For those of you still having trouble, or feel that you're not doing as well as other players, try this little experiment.
Find a person (or persons) on your team that tends to do the best, be they simply a great killing machine or capturing points/intel/whatever. Stick with them as much as you can, and try to be the best at teamwork with them. Watch their back for Spies, hit their targets with your own attacks to back them up, and take hits for them if necessary. It is my belief that through aiding the stronger players, you will not only gain experience in playing your own class alongside teammates, but learn from how they play their class so that you can put those skills to your own use. Not to mention that teamwork is key to victory, so you will likely find your team doing better because you are actively working with a partner towards the goals rather than with the team as a whole working towards kills, dominations, or defense. On their side, they get a loyal ally to help them do their job better, faster, and safer, whether they know it or not.
Most importantly, have fun with it and keep practicing! Even if your team is being steamrolled every round, or completely shut out by defense, you can still learn from the way the enemies play and the way your teammates play. In no-win situations, not only can you dare to try new tactics, but you SHOULD try out new ideas or get practice with things you're not good at. If you find that you pick it up quickly, you could even turn the tide and set your team and yourself on the path to victory! Never give up, and don't be afraid to rage a little. I do it, we all do, just remember to channel it back into your enemies through your bullets, and not through your microphone or your chat.