[burfog]

OK: got more kids than fingers, and no amputations yet!

You may be dealing with toxic friends, failed perfectionism (can't meet a possibly self-imposed standard so just give up entirely), social rejection due to cluelessness, depression, an inability to picture a future self and work toward satisfying the needs of that future self...

The kid has also had 15 years to learn how to manipulate you.

Give it your best shot, but be prepared to let go. Besides the possibilities of "not your fault" and "you were awful", there is the "ordinary imperfection". At some point, sooner or later, you have to kick them out the door and hope they manage.