I have 200+ hours in this game, and very good at handling stress irl. But i tell you what, adrenaline and stress will always smack you right in the ass as soon as those first bullets go wizzing by your head. You have to realize thats good though, its fight or flight and those reactions lead to you performing better. The only thing i could say about relieving over anxiety is to thoroughly prepare by scouting and running through scenarios. Try to always have an idea what you are getting yourself into before you go balls deep. Having the jump on someone will win you 90% of all firefights if it results to that. Dont be afraid of failing, it happens. Just think about what you did wrong and what you can do next time to turn the tide.