"It was the lowest thing I could've done in relationship terms to Rachel... I absolutely regret it, and I'll say it a million times," Tino told Nick Viall on Thursday's episode of "The Viall Files" podcast.
"It was the biggest mistake of my life... I was a piece of sh-t," he added.
Tino said he had no doubt Rachel was The One -- and that she was "the coolest girl ever" -- when he popped the question.
Tino said Rachel made him feel "really special and valued" after they got engaged. He said she was "everything [he] could have asked for and more" but the distance and secrecy of their romance "was challenging."
"We found plenty of time to text and talk on the phone, and it was great. Our meet-ups were always very passionate and we got to learn so much more about each other because we weren't on a timeline... It felt so real and it was so fun being with her," Tino shared.
But Tino said his dynamic with Rachel started to shift and change for the worse when The Bachelorette promos began airing.
"I can't describe to anyone how actually hard it is, watching [the show] back... It's not the Tino and Rachel show. That took way bigger of a toll on me than I thought it would. And you couple that with we were feeling a little distant," Tino explained.
"I think we both had a lot of pressure on ourselves; you feel like you're under a spotlight. Of course the leads get a tremendous amount of attention that I can't personally empathize with."
Tino said he was trying to bring his relationship back to its former affectionate and comfortable state.
"She had a full plate and I was asking for more. I think I could've been a way more supportive partner for her, seeing her through the storm. And that's where I feel I really let her down," Tino admitted.
Tino said emotions ran high and some of their talks "elevated" -- but that's "not an excuse for acting out." He said he tends to think "catastrophically" once something happens or goes wrong and it's hard for a person to talk him off the ledge.
Tino recalled how he gave Rachel the space he felt she needed because his insecurities were "spiking so hard," which was productive at first.
"I was feeling really insecure. It felt distant and she was just really busy. Looking back on it, there was such a healthier way to handle this stuff," Tino acknowledged.
Tino recalled getting "needy" and "pushy" with Rachel on how they could make their relationship better, which apparently made things worse.
"I was just feeling really insecure and self-doubting. I was hurting... I didn't want to be the guy who got engaged and then it didn't work out in two months," Tino explained.
"My mind just went to a dark place, and that should've been an alarm to me to go get more help and not lean on Rachel so much, especially when she had such a hard role to take on at the time."
While Tino was in that dark place, he cheated on Rachel.
"I want to put it to bed -- we weren't on a break. I don't know where that came from. We were not separated or anything like that," Tino confirmed.
"When I acted out, I felt like maybe we were starting to check out. We were just in a really dark place. Like, it's hard to describe because even looking back on it, it takes me to a place where, like, I really wish I just didn't do that and just didn't act out."
Tino previously claimed on The Bachelorette: After the Final Rose how Rachel just wanted to date him again and not wear her engagement ring on the live special. (Rachel, however, remembered the conversation a little differently).
Tino told Nick that Rachel's hesitation to wear her engagement ring played into his insecurities.
"That's where I really failed her. Seeing the ring... lit up my world, so any conversation around that, like, really, really cut me deep," Tino admitted.
"It doesn't justify my actions. I did what I did because I'm insecure."
Tino said he should've viewed Rachel wanting to "date" again as a way to get back to the "amazing level" they had been at before as an engaged couple.
Tino also said he should have voiced to Rachel how the ring conversation truly made him feel. He said he should've checked with Rachel that they had the same understanding about what her not wearing it would mean or represent.
"I was just really weak, and I'm ashamed of that as much as what I did a couple of days later. Rachel deserves better than this," Tino said.
When recalling the night he cheated, Tino said he went out with friends.
"I was feeling a little in the dumps," Tino noted, adding how he and Rachel were giving each other space and so they hadn't talked in a couple of days.
"I was getting attention and... I'm not proud of it and I'm shameful but I just kind of gave in. I just leaned in, and we kissed -- and that wasn't fair to that girl, either. That wasn't considerate of her feelings, at all."
Tino confirmed how the other woman "didn't want to be wrapped up" in any of his relationship drama.
"I did it and I realized really quickly [at the time], like, 'I don't know what the future holds for me or Rachel, but I know this is not who I am, and this is not what I should be doing.' So I got out of there. And it was pretty haunting," Tino said.
According to Tino, he didn't tell Rachel right away and that was another mistake he had made.
"Biggest regret, outside of actually doing it, was not telling her right away. I should have told her the day after, hands down. Rachel deserves all that time back, and I can't give it to her," Tino said.
"And all I can do is, you know, tell her I'm really sorry... At the time, it just really felt like, you know, the relationship is kind of checked out of and I didn't want to handle it, like, publicly and all that."
Tino insisted he knew at the time his kiss was "a one-time thing."
"[In] the following weeks, I took therapy way more seriously because I was like, 'Okay, why did I find this toxic outlet? This is shameful. This is not who I want to believe I am and I won't believe that's who I am.' It's a low-character moment, for sure, [and the] worst thing I could have done to somebody in a relationship who I really, really loved," Tino confessed.
Tino said after the cheating incident, he avoided putting himself in those positions again by not going out and drinking with the same friend group.
"I did all the steps I should have been doing before my mistake because then I would have had a healthy progression to get back on track in my own head and handling my own insecurities," Tino acknowledged.
"So I didn't tell her right away and time just kind of kept going along. And me and Rachel would talk every day. We kind of put together a gameplan of like, 'Okay, do we want this to work?'"
Tino pointed out how he didn't break up with Rachel after he cheated because he loved her and he felt he "owed her the truth." He suggested that he didn't want to take the easy way out.
Tino said "the saddest part" of the whole thing was how he and Rachel were "at an all-time high" in the days leading up to their "Happy Couple" visit, when he came clean about his mistake and unfaithfulness.
"That's where the guilt really started to take over, because she was just being so amazing -- the best partner I could ever describe," Tino said, adding how Rachel was giving him the affirmation he needed.
The couple even met up -- before their on-camera breakup -- and had a great time after Tino's cheating incident.
"We were rock solid, and I was like, 'I'm going to be with her forever,'" Tino said.
Rachel and Tino apparently talked on the phone for three hours when Tino initially broke the news of his infidelity.
"I didn't tell her the entirety of it, and in hindsight, I absolutely should have... I should've told her the day after it happened or the second after," Tino said.
"But at first I said, 'I kind of led this girl on the other night.' And then she was like, 'What do you mean? How?' It was so weak... and so lame."
Tino said he was being "a chicken" at first but then finally confessed he had "f-cked up and kissed another girl."
"I said, 'I love you so much and I want to get past this.' And she was very maturely, like, 'Let me think about this.' A couple of days went by... I said, 'Take a week, take a month, take what you need. I'm the one who messed up,'" Tino shared.
A "Happy Couple" visit was then scheduled, and that's when Rachel broke up with Tino on-camera. The event aired on The Bachelorette finale four weeks later.
Tino said it's "painful to think back" on the scandal and there were other "deeply personal" things that happened between Rachel and himself that he'll never talk about.
"But when you go through hard times in a relationship, it's hard to convince yourself there's light at the end of the tunnel," the contractor noted.
Looking back on his mistakes, Tino insisted he's definitely not on "Team Cheater," and he chose not to try to justify his actions.
"I do not stand by what I did at all. It was not cool. It wasn't fair to Rachel. And it haunts me daily, still," Tino said.
"It's something I'm ashamed of, and certainly, I wish I could have done it differently, like, a million times over."
Tino revealed how he's been going to therapy twice a week, working on himself and his faults.
"I'm trying to go through and unfold and figure out why my insecurities, my ego, could just take over my actions like that and just kind of lead me down this path to go to rock bottom and dig a 10-foot hole beyond that," Tino said.
Tino said he's going to "strive" for the rest of his life to disprove the phrase, "Once a cheater, always a cheater." He concluded that's "not the case" with him but Rachel had every right to dump him.