You have 2 choices really. Be positive or be negative. Between the 2, why would you choose negative? Are you wanting additional stress, depression, etc that thinking negatively contributes to? How you think may not effect the outcome, but it will help you handle the outcome in case it doesn't go your way.
I struggle with this at times. I'm in the process of getting tested for other medical issues probably created by chemo and/or rads. Scary chit. Going in to the results with a positive/hopeful outlook presets my mind to handle any potential bad news better emotionally and helps me to think more clearly the next steps. If you're negative, you go in already in that mode- and should it be bad news, you'll be in worse shape and won't be able to deal with it as well.
The only place I allow myself to "go there" is in my blog because you do have to voice and vent your fears if only for your own self. Otherwise once I click out of that board, I got it out and work my way up to being positive. Blogging also helps me when I go back and see what I was fretting over didn't happen. For issues that are ongoing, blogging is great. What I've found over my adult life is most of what I've fretted over, thought I'm doomed, never did happen. So why waste my energy on something bad that hasn't happened? I gave myself a pass after my doc appt and am not going to schedule any appts until Mon. Small (healthy) things that you can control to make yourself handle it better, do it.