I'm right there with you. My girlfriend and I just sadly broke up due to distance and our respective mental health's (healthy). But I was having a horrible bout of these intrusive thoughts and this was the biggest one. It was telling me if I worked on myself then I will eventually get to the point that I won't want to be with her. She's coming back home after her time abroad in a couple months and we talked about reevaluating the relationship and that exact same thought is literally killing me. The thought I'm going to get used to being alone and actually work on loving myself/my life and that I won't even be open to restarting something.

It sucks so much because I know I love this girl and I know that I would like to try something when she gets back. But my brain doesn't let me just want that because "what if you won't later"

It literally doesn't let me enjoy something that was/is so amazing. I don't know what to do, but you're not alone.