Just play more games, you are nervous because you think that you have to be the best when you dm so much
you have lack of confidence-develop you confidence and you will be ok
i had that problem too and i mostly warm up playing dm before an actual game and i was shaky too
play DM with only pistol first - gets rid of that shaky aim and makes u go for the head always
thn play DM with rifles
It's perfectly normal, deathmatch can only improve your aim and your crosshair placement (and even then, many positions have no duels taking place).
Because DM servers are so full, there's also the problem of shooting many people not looking at you or when they are, they won't be in the same mindset as your opponents in a real match are, because they're faced with loads of enemies, they have to react from many angles at a time, while in matches your opponents very often expect the duels they take.
If you want to practice more efficiently, you should try retake servers for example, where you can practice real situations which are far more likely to happen during a match. There are also some duels servers, which can be a bit useful as well when it's made on official maps, even if it's only a series of 1v1.
But nothing can give you a more efficient practice than real matches themselves anyway, because if you don't have the same stakes, you won't play the same way. Obviously take advantage of the replays to learn from your mistakes.
Also, obviously, practice your nades and know your maps, know the angles, timings, wallbangs, etc... And don't study everything at once and try to apply everything. Try some nades, use them until they become second nature, then add some more, etc...
If you're not on a soloQ quest, you can also find reliable teammates on which you can depend on to play regularly. It's easier to have a consistent level when playing with the same teammates than just solo queuing.