Xanax (or other benzodiazapenes) won't do it by itself, but it does potentiate narcotics or methadone, that's a pretty sure way to check out, and painless, too. Just 'cause you asked.
Seriously, dude, you're just chemically depressed from abusing the gak right after coming off the 'done, which will depress you in and of itself. The mood-creating part of your brain ain't firing on all eight cylinders right now. Give it some time. Get some good food in you, try to rest, smoke weed, and stay off the hard shit for a while. Try taking vitamin supplements and eating regularly. If the depression keeps up after a few weeks, then make other plans. Suicide's not a decision to be taken lightly--I think you should give yourself at least 90 days to think about it first. Mine has been postponed indefinitely for a while......