I don't think being a "what if" thinker is entirely a bad thing. It can help you dealing with worst case scenarios or calm you down.
For example, I used to get pretty nervous a few days before exams, thinking I wouldn't have the time to study everything and that would mean a bad grade. So I would think, "What if I get a bad grade?", "I repeat it one month from now, I have two other exams but I can rearrange my schedule, it's definitely doable", and that would calm me down. Or I would think that I could make up to it by having higher grades in other subjects.
I have to say this kind of thinking allowed me to deal better with changes around me, I can quickly come up with worst case scenarios and how to deal with them, which calms me down and allows me to take care of the issue better, which then leads to the worst case scenario not happening. But if it were to happen, I would be ready for it.