I experience something similar if I spend too much time dwelling on things -- I'm guilty of 'overthinking' and it almost always leads to imagining (exaggerated) negative scenarios and outcomes. I also run into trouble often if I do too much reading on the subject (including here), because I'm quite sensitive to anecdotes regarding negative or sub-optimal outcomes or problems that others are having with their transition. I have to be careful or else I end up getting really down and feeling like things aren't going to go well for me either. As I'm sure is the case for all of us, my transition is so important to me that it's easy for me to get super anxious about progress/outcomes/social aspects and it can really damage my mood and my self-esteem.
All I can do is try to identify times when I'm upsetting myself (or when I'm likely to upset myself if I continue). If I'm already down about something, I try to remind myself that my negative emotions are just a function of a bunch of my own thoughts and projections that have nothing to do with reality. However much they may seem real or inevitable to me at times, they're really not. However rational they may seem, they're ultimately baseless and often irrational. So I try to rein myself in before it gets to the stage where my thoughts and feelings are out of control.
Of course, I don't always succeed with that. ^^