The way I see it, you have three options:

1) Avoid endgame. Do stuff once it's a couple patch cycles old when everyone outgears it and can carry you and you messing up isn't a big deal.

2) Accept that you might not be the best Black Mage out there--and then do what it takes to change that until you can be confident. Talk to other BLMs, look up guides, look up ways to be able to judge your performance, practice rotations on dummies and get good enough that when a group wipes you KNOW it isn't your fault--or if it is, you know you can handle it next time, now that you know what's coming.

3) Quit.

Personally, I suggest option 2.