Amber Portwood and Matt Baier have been plagued by rumors of why their three-year romance is often on the rocks, and now the Teen Mom OG star herself is explaining why she called off their wedding.

Amber, 27, admitted Matt has made mistakes in the past, but there's one issue she won't let slide: Substance abuse.

Amber explained that she caught Matt offering her Teen Mom OG co-star Catelynn Lowell a Xanax to calm her nerves during an April press day for the show, Us Weekly reported.

Amber was apparently furious Matt, a 46-year-old recovered drug addict, even had medication on him to begin with, so she called off their wedding that was initially scheduled for October 13.

"It was pretty much the straw that broke the camel's back," Amber, who is five-years sober, told Us. "We're not together now."

However, sources told E! News that Amber and Matt will appear on Marriage Boot Camp: Reality Stars Family Edition together to hash out their issues, along with her mother Tonya Portwood.

Amber didn't confirm her Marriage Boot Camp participation when speaking with Us, but the mom of eight-year-old Leah confessed, "I think therapy in general would help."

Amber said the couple would need to work on trust, although she is convinced Matt was never unfaithful in their relationship.

"I don't believe he's cheated on me -- [but] no more lying, no more verbal attacks against each other," Amber said.

"There's a lot that needs to be worked on for us to even think of being together. He hasn't been the best. He's just met a person that doesn't deal with his sh-t. But he's also met a person that he really is in love with and doesn't want to let go."

Amber is therefore waiting for Matt to step up his game and fight for their love, sharing with the magazine, "At this point, I told him it's pretty much up to him to save this relationship."

Marriage Boot Camp currently airs on Friday nights at 9PM ET on WE tv, while Teen Mom OG airs on Monday nights at 9PM ET/PT on MTV.

Reality TV World is now available on the all-new Google News app and website. Click here to visit our Google News page, and then click FOLLOW to add us as a news source!