Try other modes like ARAM or the featured ones on rotation? You're gonna get salty people in any mode - that's just the nature of the game, but people tend to be much more tryhard on the Rift (even in normals) than they are in other modes.
If you aren't confident in your ability to do well, then practice the areas you think you may be criticized the most in. CSing with our without lane opponents can easily be done in a custom game with all bots. ARAM is actually a good field for teamfighting in general.. only drawback being you don't really get to choose your champ or have a "normal" teamcomp.
In the end though, the only way to get better and to improve both your play and your confidence is to just keep playing. Who cares if some random people on the internet take a casual game too seriously? You'll probably never see or speak to them again so what does it matter? Yes, I understand that this is the same mentality that people who intentionally troll have, but if your intentions are good and you are trying your best, then play with the priority of enjoying the game for yourself rather than trying to please others. You'll also need to have an open mind to accept constructive criticism if you do make mistakes. Unfortunately, people tend to flame and complain more than they offer advice, but there are still good people out there.
