if you feel you have to be perfect or as close to perfect as possible, think again. nobody is perfect so please give yourself a break; you're not perfect and neither is the person who "let you down". you're doing your best with what you've got so why add negative self-talk into the mix?
a lot of the things we say to ourselves, we would never say to someone else. I've been curious about why that is; as they say, "we are our own worst critic". I started working on this for myself years ago and the way I got to stop (or at least limit) it was to pretend I was talking to someone else. do you talk negatively to yourself at times?
the next time you find yourself having a negative self-talk, remember that you're not perfect, hahaha. give yourself a break and remember how amazing you are, flaws and all; and in case no one told you today - you are worthy of love, peace, and happiness. now go out there and be great!