It looks like my gear may be part of the problem; it's still the Mythrite stuff. I'll see what I can do about increasing that and see how things improve.

That being said, I'm having this issue in lower level Guildhests that I'm doing for practice. I can't say if there's a specific type of class that gives me the most trouble. At this point I'm certain it's something I'm not doing, or doing incorrectly, and not the other players' fault. I've successfully maintained aggro on some mobs when doing FATEs with FC members, but after seeing my poor performance, I'm hesitant to continue with dungeons with random people until I'm up to par.