For me, my nerves were definitely much worse when I first started playing, but as I played more, improved my skills, and held higher combos, I found out that I have somewhat become more desensitized to it, (and honestly, I kinda miss the excitement and the adrenaline rush, maybe that means I should try to fc harder maps lol.) I realized that stressing out about this game does not make me play any better, so I am able to stay calm better just with more practice, and familiarity with the game, and just getting more confident and being able to trust my own skills and abilities. Also, before I was an osu! player, I used to play gd, a game with a even smaller margin for error, and something that I learned when playing gd, that also transfers to playing osu!, is that a lot of the stress of holding a high combo, or being pretty far into a gd level, is you don't get these oppertunities very often when you are playing. So the stress (at least for me) comes from not wanting to mess up, knowing that if I do, I would have to go back all the way back to the beginning, in order to do this all again, which I might not be able to, for some time. So in order to deal with this, and make these oppertunities feel less rare, I would practice the map until I feel that I could play it pretty well pretty consistently, and when I am having a good run, and I get stressed, I tell myself that if I fail, or break my combo, it's not a big deal, because I know that I can get back here relatively-easily and try again, until I actually do it. There is really no need to rush, you can take as long as you need to, and something that helped me improve in osu! is that when i find out that I am getting tired, or frustrated when playing osu!, and that I am no longer improving, or even playing worse, I would take a break from that map, maybe for a few days or something, and I find that more often that not, when I come back feeling refreshed, I would actually perform better than before (I think my brain might've been subconsiously practicing when I was away, because I get mopre dreams about playing osu! than I'd like to admit lol.) Also I know that this is kinda unrelated, but if I am able to hold a decently-high combo and then I choke or break combo, I would still celebrate the play, especially if it is my new top play for that map, because it means that I played well for the majority of the map, and I am one step closer to actually getting an fc in the future.