How to rebuild trust?

My husband and I both have drug problems in our past (mostly separate, but at one point together). I quit over a year ago, and he has been trying, but keeps slipping. He has continuously lied about it until recently. I threw him out and told him it was over. He is now in treatment. I want to believe in him and trust him - we have 7 years together with almost 2 years of it being married. I want to be with him so bad, but I don't know if I can ever fully trust him again. Is there any hope? Or is it a lost cause that is just leading to more heartache?