Just have some fun man. Do some bgs, do some 2s, some 3s, etc. I mained my warrior and played him manually. Took a lot of bgs to get good at it though, but as long as you don't stress/get worked up about losing you'll get better in no time.
You'll only get better at a class with practice. What I did for my warrior was learn the rotation by hand. (this improves situational awareness like crazy when you don't have to think about what to cast next). Practice your rotation on dummies/bgs/duels, etc. I dueled a lot in MoP and that gave me knowledge on how I could counter classes on my warrior. This really teaches you how other classes set up their burst, and how you can shut it down. There are situations where you can fear a rogue, kick a mage on frost, and stun a druid, so the rogue can't be dispelled. and give your teamates that few seconds to recover.
Another thing you need to start doing is anticipating the next incomming CC. LIke if you miss a sheep on your healer and you're fighting a rmd, start hitting the druid to stop the clones. This all comes from playing alot of games. I had about 2k games played on my Alliance warrior and 1k games played on my horde warrior in WoD alone.
What you should do is record your games so after every session you can take a look at your gameplay and figure out what you could do better. That's how I went from 0 - 2500mmr on my warrior in less than a year.
Another thing, don't blame your teamates for every loss. If you start recording your games, after every game you'll see what you could've done better to end the game faster/best ways to peel for your teamates, etc.
Edit: I really want to open a new account right now lol.