I feel your pain, I really do, but don't give up
I'm a medic main who's very new to the scene (been on this site since December 2013) and I also occasionally get flamers and ragers. Best thing to do is to ignore their abuse, mute anybody particular repugnant, take any constructive criticism that you get and just try and do your best.
You need to get used to people raging at you to a certain extent though as medic is possibly the most important class in the team (along with the demo) and so when you mess up, the whole team will know about it. It's also easy for bad players (especially pockets) to blame you for their mistakes as well.
I suggest that you try and make some friends with better players who seem patient/nice and/or willing to help you. I've made so many high division friends on tf2center and some of them have been instrumental to my progress. This is also a nice way of finding players to play mixes, scrims and even form teams with. These are even better than lobbies for practicing medic as they'll always have better comms and better players in them, plus there are a lot of decent, high division players in tf2center you'll end up playing against at some point.
Also always join mumble regardless of whether it's required or not. Even if you can't use your mic for whatever reason it's useful to listen out for calls of enemy positions and to listen for people asking for heals. If people in your team are not in mumble then sometimes asking for them to join mumble in in-game chat will get a few people joining. At the very least you should aim to get your pocket in mumble because he will be protecting you 90% of the time. Demo and roamer are also very important and scouts would be nice but not as important in my opinion.
If you really want to become better at comp tf2 then it'll take a long time but it's a very rewarding experience in my opinion.
Also here are some great guides/general resources on playing comp tf2 medic. It's very much an art form of it's own:
http://steamcommunit.../?id=163882605#
http://etf2l.org/help/new/
http://etf2l.org/for...c-15054/page-1/
http://etf2l.org/for...cs/topic-12510/
Hope that this was helpful for you
Oh and you can also view your stats for each game on http://logs.tf/ or just google "logs.tf". If you can't find your latest match then look for it on sizzling stats (http://sizzlingstats.com/) as it can be on there sometimes.