Also trying to respond to any and all criticism, failure, rejection, negativity and annoyance and injustice with stoicism.
Hey I'm guessing you mean stoicism in the classic Roman sense, not the modern day meaning.
This is very difficult as you noted. The best advice I can offer is (A) a true sense of self worth grounded in whatever "does it" for you, so you can then use your (B) iron will to focus on it. Seems like failure and rejection and negativity is all over the place, and starting out trying to just let it slide is rough. But it does get better.
Much of the classic Roman stoic techniques were, imo, ways to prepare yourself mentally for an inevitable defeat/rejection/etc, so when it does occur it's just business as usual. It wasn't a way to just overwhelm yourself with the shit of life like the cynics, but rather to get on with your life knowing that you'll be fine when it goes bad.
There is also a large overlap between some of the Buddhist teachings about suffering and the stoic middle ground. If you care about rejection, and failing, and negativity it's going to affect you.
Oh, and if you're scared of rejection from girls specifically, just go approach 20-30 of them one day, hit them with the shittiest line you can think of. Make a game of being blown off publicly and you'll lose your fear pretty fast. Which now that I think about, inoculating yourself against bad situations by putting yourself in them, was a stoic technique espoused by Seneca.
Yeah, this wasn't about ethics, but that term is so loaded and relative it's hard to discuss.