Scared my boyfriend will leave me for another woman. am I thinking rationally?

Anonymous
I have been dating my boyfriend for a little bit now. Before I started dating him, when we were just friends, he had a crush on a girl like 1 or 2 years younger than him. He's liked her for many years, but never bothered to turn it into anything serious. We'll call her Melissa. A little insight on me is that I have been diagnosed with Borderline Personality Disorder (BPD) and Severe Anxiety.

I've been super upset and dealing with super obnoxious intrusive thoughts in regards to the two of them. They don't hang out in person much (only on occasion) and I have no idea if they actually talk everyday anymore, maybe they do. I'm away for University so we're doing Long Distance at the moment. I'm just worried because what if he ends up falling in love with her if they hang out again? what if he truly does not love me? what if he still has feelings for her that he's choosing to surpress? he follows 3 of her accounts on Instagram (private, fun page and a main page) and likes her pictures. I know I have jealousy issues but I've been working on overcoming those. I want him to basically remove her off of everything, but I think that'd make him leave me. I want to trust the two of them together and remain a supportive girlfriend of the idea they're just friends.. but he loved her for so long, before he met me and when we were friends I tried to convince him to be with her but he chickened out. I love him, and I don't want to lose him. Am I irrationally thinking or do you think you'd want Melissa removed from his life as well if you were in my shoes?
*serious answers only please*
Scared my boyfriend will leave me for another woman. am I thinking rationally?
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