When I feel down about my progress, or about an event coming up that may derail me from my goals, or if I get injured, it's extremely discouraging and makes me want to give up. Instead of letting my pessimism take control, I turn it into a challenge! If I pull my hamstring and can't run for a week, I will challenge myself to do more strength training. If I'm attending my best friend's wedding, I take on the challenge of eating properly and dance my heart out to burn those Calories!
Bottom line: If you think your circumstances are the end-all of your success, they will be. Realize that if you overcome the hardest and most difficult moments, you will only have a reason to celebrate more in the end!