Hello, many people feel like you at times. Usually when a team can't make it or has a lot of problems, it's not just one member's fault. Tanks and healers are the most visible, that's why less people play them.
When I did Qarn for the first time as a PLD I was not good. Anyway we won (with many wipes on 1st boss, a couple on last.) I repeated for practice thinking I would do better and second time it was impossible to pass the first boss. Wasps would always kill me, somebody died by doom, etc. Next try was better.
I feel very bad when I think I am ruining other's runs. Many times I choose not to do high level DR for this reason. If you are unlucky with the team, you may blame yourself, even if other times the run was smooth and easy.
Most people are very nice. Funny when one blames another and is rude, the one blaming usually is not playing well and has a lot of fault. Try not to get too upset if it happens to you (difficult, I know.)