Hey man, just know you're not alone in this. We're all really passionate about something, and sometimes that leads to us getting frustrated if it's not going our way. It's normal, and a stepping stone to being able to process your frustration better.
What I noticed with myself is that I expected WAY too much of myself in a super short period of time. I expected to excel within days when something like that could take some years to accomplish. But as cliché as it is: Rome wasn't built in a day. And you won't be able to become a pro soldier in a day either. But every day you practice is another day closer to that goal.
Next time you get frustrated, take a breather, move away from your computer for a minute and just walk it off. Definitely give other classes a go too. Not only because it's something fresh and new for a chance, but it'll also give you more insight into the game so you will in fact become a pro some day.
Also don't main medic. Worst choice of my life.