In running, like most other endeavors, hard work is the foundation on which confidence grows. Every workout you complete, long run you log, and sacrifice you make to improve as a runner should make you feel good about the progress you're making. Before you toe any starting line, you should reflect on the work you've put in to get to that point and derive confidence from the sweat, suffering, and commitment that has gone into your preparation.
Better yet, keep a training log so you can keep track of every run, cross-training workout, strength session, and exercise class, and remember them when you're getting ready to put them all together in a race. I've been keeping training logs since my first cross-country season in high school (2007), and I love to flip through them all, reminiscing on breakthrough workouts, character-building slumps, and especially memorable races.
If you're not the pen-and-paper type like me, an Excel spreadsheet does the trick, as do online logs like Strava and Running2Win.