I was kinda scared of healing Pharos Sirius simply because I was worried that I wouldn't be able to keep up with all the heals required on the last boss. Yesterday, I grew a pair and went in to do it. Granted, we didn't get past the first boss (tank would not take aggro, we wiped) and they all left. Didn't get far, but I felt confident I could do all of it. Playing Healer, I am out of my comfort zone (I main SMN) so before every dungeon I'd make sure to at least watch a video so I know what to expect. That way I have a better chance of doing my job properly.

As for people shouting at you/getting annoyed etc, ignore them. I always do. If they rage at me, I just ignore them. It's a lot easier that way. For games like this, you do need a thick skin, since it is full of children who will try and make you feel bad/annoy you. Just ignore them and the game is a lot more fun.