I know it might sound a little too simple to work, but take a slow, like 3-5 seconds inhale through your nose and then exhale forcefully through your mouth. I found it on the internet and it really helped me. The other thing is just your mindset, for me it was easy since I never get too excited if I get a sick clutch or a kill, so I don't really know how to help you with that, you've got to work it out yourself, but my point is, specifically while clutching you shouldn't think about your teammates that are watching you and being afraid of making mistakes, don't get too excited, just try to make as much damage as you can. I remember back in 2014 after clutching 1v5 with a p250 vs full-buy my hands were shaking for the rest of the game, I fixed it by playing a lot, had so many sick clutches and just moments that I hardly ever get excited nowadays. Also, you're panicking because of adrenaline, it increases your reaction time and does some other stuff but also has it's bad sides.