i used to struggle a lot with being nervous during matches. as you gain experience and play through it, you'll gradually learn what works for you. this is always going to be a very specific and personal issue that you need to address yourself, but hopefully the responses in this thread can give you some things to implement into your own solution.
i'd encourage you not to follow the advice telling you that it's just a game and that nothing matters because it isn't going to help you become a better player. if your justification for playing poorly due to nerves is that it doesn't mean anything, then you're not a competitor and it's just illogical to spend your time on something you aren't invested in. accept the fact that you are already spending your time playing this game -- not putting your best effort forward would just be a waste of your time if you are playing for the sake of competition. if you're going to do something, do it well.
if your main struggle is being too jittery to aim or talk, do whatever you do to relax beforehand. it will never completely go away, and being nervous means you are focused on the game, which is a good thing. it's just a matter of using that feeling to your advantage, rather than letting it control you. I find it helps to go into your match with a plan that you can stick to. distracting yourself by making it about the game and focusing on what's going on in the match will take pressure off of thinking about your aim, for example.
after your games, spend time thinking about your failures and what the underlying reasons for them are. instead of brushing off a poor performance as you not playing well and being nervous, watch the demo and consider what decisions you made that contributed to your performance. i've never seen a player perform poorly and make good decisions at the same time.
one of the things that helped me the most was making it about the team and trying to always be the kind of person your team can fall back on when things get rough, never shutting down. if your team is down 0-3 in a match and people are starting to shut down, be the person to question what's going wrong and make decisions to address it. if their roamer keeps killing your medic, instead of saying "our medic keeps dying", phrase it as "we need to keep our medic alive, can you stay closer to our medic scout?". being afraid of making decisions is worse than making poor ones. even if you're not the main caller on your team, you need to be capable of analyzing what's going wrong and offer that information to your team.
in a tournament setting where you are playing multiple matches or just long matches in general, people tend to lose focus once in a while. if you're noticing your teammate start to zone out, be the person to try to bring them back into it and coordinate with them, because you need them to do the same for you. i can't tell you the number of times i've started to go into autopilot during a match and have b4nny ask me "where are you taking the uber? what do you want to do?" to get me back in the game. little things like that can go a long way.