ive been speedrunning celeste forsaken city. But every time im on a run with good pace i choke up and make stupid mistakes does anyone have tips on how to keep my nerves down?

You can try to practice segments/levels of the run you're having trouble with first until you get to a point where you're confident enough in them. With enough practice and enough failed runs, you'll reach a point where you either get confident enough in your abilities to not fail or you straight up just do them mechanically, as they'll feel as second nature to you. Skips I wouldn't initially have dared to do now I just do them like it's nothing.

Also, allow yourself to fail. If you catch yourself trying to get a perfect run at the first attempt, you'll be often disappointed/angry at yourself when you make the tiniest mistake. Just start finishing runs and your PB will slowly but surely progress to lower times, little by little, and seeing that progress will put you in a better mindset over the long run.

Once your PB gets low enough, you'll be in a position where you have mastered most of the basic mechanics and you'll be forced to advance to riskier strats, which will make it so you don't second-guess yourself all the time (ie. since there's nothing else but to do the risky strat, you'll just go for it without giving it extra thought, it'll be "do or die").

If you're still not feeling it, you could also consider taking a break from speedrunning the game (or speedrunning as a whole). I've found that often times when I pick up a speedgame after a few months of being on a break I get considerably better runs, given that I'm fresh and all the practice I previously had comes back after a few derusting sessions.

In any case, you gotta find what works for you, and more importantly, how to make the endeavor as fun as possible for yourself. It's a hobby, not a chore :)