Which would you rather have - regrets or mistakes?  We spend a lot of time not doing things because we are afraid of failure.
We focus on the potential for error, on not getting that job we are thinking of applying for, on what might happen if we fail. You know what? It becomes a self fulfilling prophecy.  If you do overcome your fears, and put in the application, you have already convinced yourself of your inadequacy, rehearsed the lack lustre performance at interview, and given yourself permission to fail.
How about approaching it differently? What if you were to rehearse the scene with you as the successful applicant, and imagine the interview with you performing at your very best. Imagine the body language you display. Back straight, shoulders back, the interview outfit immaculate and demonstrating your absolute command of the room.
What if you did the homework beforehand, and had good questions ready for the end of the interview (not the ones about pay and leave! The ones that impress will demonstrate your research into the company, your understanding of their place in the market, and how your role fits in.
What if you set out with the express belief that the job was yours and you not only deserved it, but were the best possible candidate.
How you approach your application has a huge impact on the outcome. 
So - regret that you didn't apply for that promotion - or risk being unsuccessful.  And if you don't get the job at the first try, ask for feedback, find out what you could do to improve your chances next time.
Don't wish for the stars...reach for them!