"I'm sad. I cried a lot... I was completely blindsided [by the breakup] and it just happened very suddenly," Amanda, 29, told her fans.
Amanda met Bobby on Instagram and went public with their relationship in May 2018.
Amanda seemed perfectly happy in recent days and even told Us Weekly earlier this month Bobby was "a father figure" for her two daughters -- Charlie, 5, and Kinsley, 7 -- and she hoped to "get married and have another baby" with him in the near future.
"First of all, I want to let y'all know I am totally fine," Amanda insisted in her Instagram Stories. "I wanted you guys to hear it from me that yes, Bobby and I broke up."
Before getting into what happened, Amanda wanted to make one thing clear.
"First and foremost, I want to clear the air about my kids. My kids know, no matter what, they are always my number one priority. Always. I remind them that every single day," Amanda explained.
"They never feel like I'm choosing a guy over them, and that's the silliest assumption about me that I've ever heard. Me and my kids have such a special bond and they know nobody will ever come before them... They are the happiest, sweetest, most-loved kids in the world."
The Season 20 The Bachelor bachelorette, who also appeared on two seasons of Bachelor in Paradise, then shared how her children have a very involved dad, her ex Nick Buonfiglio, in their lives and so she's not looking for a replacement father when she dates.
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"Secondly... I wasn't ready [to talk about it before] because it's super fresh, but I was completely blindsided [by the breakup] and it just happened very suddenly," Amanda continued.
"Before I get into it, Bobby didn't do anything wrong, so don't go hating on him for it. Yes, I was surprised, but it's okay."
Amanda revealed she and Bobby "weren't fighting" and they actually had "a great relationship," which is why she was so "shocked" by his decision to dump her.
They had also just recently moved into a beautiful Laguna Beach home together, which apparently was a huge deal for Amanda.
"I thought we were going to get engaged by the end of the year... so I was very surprised," Amanda reiterated.
"[Our relationship] wasn't toxic. It was actually the healthiest relationship I've ever been in. He was my best friend, I was super happy and that was not fake. I'm not the type of person who fakes things for Instagram... I want to be real with you guys."
Although Bobby seemed to pull a fast one on Amanda, the reality TV star said she respects his decision because people are allowed to be happy and change their minds.
"I'm glad he didn't waste any more of my time," Amanda reasoned, adding that it was also nice Bobby didn't continue to lead her on.
"Me and the girls are going to keep the house we moved into."
Amanda ended her message by thanking her fans for their support and confirming there is no chance she and Bobby will reconcile.
Amanda apparently doesn't believe in breaking up and getting back together.
"I just don't think it works out," Amanda noted, adding that she wouldn't want to be in a relationship in which she fears getting dumped when everything seems to be going well.
"That's not something that's going to happen, and that's okay."
Amanda is also proud of herself for being so "strong" and focusing on herself during this difficult time.
Bobby confirmed his split from Amanda on Friday, several days after rumors of their breakup swirled based on their social media activity.
Bobby insisted to a fan on Instagram that cheating had nothing to do with the demise of his relationship and he'll "always be there" for her two little girls.
When another Instagram user asked Bobby if he left Amanda over the recent hacker scandal in which a man allegedly obtained topless photos of her from her doctor's office and has been blackmailing the reality TV star, Bobby said, "Not at all."
Prior to getting involved with Bobby, Amanda had a highly-publicized breakup with her ex-fiance Josh Murray and then a short-lived relationship with Robby Hayes, both of whom had appeared on different seasons of Bachelor in Paradise.