DAE have extreme anxiety regarding their relationship?

 My partner is everything I could possibly ask for but whenever he's not around my mind starts spinning towards "What if we break up?" (Not something logic like "I want to break up with him" because I don't-- it literally takes the form of "What if this happens out of my control" for some reason).

Being inside my head so much is also ruining my experiences with him since I can't really enjoy anything in the present moment. My mind is always ten steps forward, and not in a positive way. When I reach out to 'normal' people (which means non-anxious people or people without a medically diagnosed mental illness like BPD or anxiety) they always jump to the conclusion 'Well then maybe he's not the one'. But I *know* they're wrong. I know this is something in my head and whenever I tell my friends about something 'upsetting' he's done I also know it sounds worse than it actually is because my anxiety-ridden mind is ballooning everything out of proportion. 

He's never done anything to hurt me and has always been the sweetest person. And I don't question his motives or anything... I just worry that *what if this ends one day*.

And I know it will, because either we're going to break up some day, or eventually one of us is going to die if we're lucky enough to grow old together. But I have this sense of impending doom, like it's going to happen *right now* (again, *not* the feeling that *I* am going to dump *him*, but rather that it's just going to happen naturally... In some weird way). And these feelings always pop up when he's not around.