Try to start small and work your way up. Get in combat with a small mob that won't hurt you and just stand there. Does your main ever just randomly target himself? No, move on. Next push what you do to attack (on your main only and with no weapon, or you just gank the mob). Does he target himself at any step? If he does, then stop and check what the last step he did that made him target himself. Things that can do it: /target, mouse-clicking, F#, probably F1--I think that targets the party leader, which is probably yourself, I mapped F1-F5 off party people and onto my Fkeys, and about 10% it could be using an ALT modifier with an item, like ALT-bandage, it bandages yourself, it might try to target yourself. It shouldn't, but maybe something is working wonky.

Basic idea is try to reproduce it repeatidly and you will probably find out what is causing it. That is 50% of what boxing is, finding out what is wrong or what works. Just takes time.
Good luck.