I stopped playing MMO's a long time ago. Other than a lot of them having micro-transactions, the player-base can sometimes be cancerous. Whilst SOME single-player games have opted for micro-transactions, that's a small minority.
With single-player games, it's just you, so you don't need to stress about being judged for your ability. You can take time to learn different things, and play how YOU want to.
There's also a variety to choose from, and you can be in control of how much you want to play, and when you want to play. So if you have exams, you can stop playing games for awhile, and not have to worry about your group of friends raging for you to be online. You could also play for half-an-hour one day, and that'd be completely okay too.
You don't have to if you don't want to, but this is a good alternative to your problem- you can still game, but it's more flexible with your schedule and you choose how you want to play without limitations.
Something else you can do is leave your friends' group. If they're giving you a hard time, blaming you and making you stressed, you don't need to do anything for them. Leave, find a nicer party that are understanding, and play with them. You'd be surprised how many cool and great people you can find online.
But one thing is clear- stop playing with these people; whether that means stop playing MMO altogether and go to SP, or find a new party.