I think I know what you mean, although you seem to have a more severe case of it than me. Kind of a fear of rejection I guess, or maybe just a general negative bias going on in our minds. “The worst possibility must be true, other possibilities are just wishful thinking”, something like that.
If the thoughts start, you could say to yourself, “No, I’m not going to think about that. I’ll think about something interesting instead”. Then start thinking about a hobby of yours, or anything that interests you. Alternatively, read, watch or listen to something good.
So basically, try and stop the negative thought train, and replace it with better thoughts. That’s what I try to do.