I know very well what it's like to be in that position. You love them so very much that you can forgive anything... The problem with that is that they tend to walk all over you when they realize you'll forgive them so extensively. Forgiving them doesn't have to mean doing nothing though. If he's into drugs and drugs cause him to act destructively, it's no loving act to simply forgive him and do nothing. He needs help. Rehab. Sure, forgive him, but get him the help he needs and insist on him doing it. You never know. He may leave you no choice but to divorce him. You can try to get him help and stand by his side through it, but if he's unwilling to make an effort to honestly change you can't just sit there and watch him destroy himself. That's my opinion anyways.
It would be easier if you didn't love him so much, but if you didn't have the love and commitment you're showing, you would have married for the wrong reasons. Walk away if you don't have any other choice, but if you do walk, walk away knowing you did everything you could.
Habits are, well... habitual. People with addictions tend to fight those addictions their whole lives, what ever that addiction might be.
I hope the best for you both.