Little rule-reading! problem.
Welcome to the forums!
So uh... It seems like this might've been posted in the wrong place. Judging by your topic, I'd suppose this just goes down in Gameplay & Rankings, which I'll leave the links to over here.
(OLD: https://osu.ppy.sh/forum/13) (CURRENT: https://osu.ppy.sh/community/forums/13)Anyhow, remember to be reading the rules of each forum to get an idea of what the forum or sub-forum is for. That way you can efficiently navigate the forums and post in the right places.
As for the actual problem you have, I don't really have much to say on it. Most of the time it's best to endure and stay focused and calm. Your muscles and nerves may be aching for that win but dont let it get to you and just do your best on the beatmap you're completing. Remember to stretch your arms before and after playing difficult beatmaps, which can help get rid of whatever stiffness might be going on. That's about all I can say to do, as learning to control your movement is trial-and-error, and everything takes time and practice.
Good luck!