I know some great tanks who just never totally became confident with that role. Some people just dont have thick skin for dealing with jerks, and so these tanks end up only wanting to tank with friends, rarely venturing out with randoms.

I personally have a grand ole' time with jerks in DF, but that's not for everyone. And it doesn't matter how good or bad you are as a player, someone will still try to make you feel like crap--it's hardest on tanks not because tanking is harder than the other roles, but because our mistakes stand out the most, making us targets. The best thing you can do for your confidence, IMO, is just make sure you understand your skills inside and out, and make an effort to understand the game itself (other roles, etc) so if someone calls you out on something, you don't have to be uncertain of yourself, you can simply state what happened and why. If it's your fault you can apologize (unless they're jerks, then try to subtly kill them on the next pull without wiping the party), and if it's not, you can stand your ground and call them out.