the above advice is golden,
i have been on and off all my life (all kinds of drugs)
and i have to say, you can still go back to college, grades dont mean crap, your old ones i mean. what you do next is what matters, you can go back to college.
You can do with your life what You want, just knowing you have a problem is the first step to a cure.

Good luck bud, i really mean that, pot is not everything, sex is not everything, life is everything!