John, I agree with Dee's suggestion: talk to the counselors at your college/university. They will be able to help work out a manageable academic load while you deal with your addiction. Think of it as an illness, no one would expect anyone who is sick to be able to study effectively. Drug addiction is very common in college, they may be able to put you in touch with a support system/program like NA.
I made the Dean's and Chancellor's Award list at university while dealing with an opiate addiction, so it can be done and you're not alone. University became a strong part of my recovery, it was a refuge.