I have this issue as well. I over think things regarding my relationship often, and tend to think of the worst possible things that could happen, which makes me anxious.

When I do this, I breathe deeply to try and relax, and say to myself that I'm thinking of the worst case scenario, now what is actually likely going to happen, or likely happening? This tends to help, because as I said I'm usually thinking of some far-fetched worst case scenario, and what is likely to happen is usually something that is nothing to worry about.

I hope that made sense, I know I was kind of rambling

Good luck, I know how difficult this can be to deal with.