You can cling onto the hope without marrying him. Marrying him will have the opposite result, he will not get better, he will get worse. It becomes a game of the mind, he says to himself, I've got her, she married knowing full well that I was an addict, what did she expect? You say, I married you because you PROMISED to stop using, you didn't. Once married to an addict you have given away all your power.
That piece of paper you so desire is nothing but a financial agreement, once you sign on the bottom line, the debts he racks up in the future also become yours. If he drives while stoned and gets in an accident, the monitary consequences now become yours as well.
To me, you are playing with fire, try and not get burned too badly. If you do go forward with this marriage, please consider not comingling any funds, set up one checking account to cover the household bills and keep the rest separate.
Read other posts here, many are losing their homes, filing bankruptcy or are in real financial straights, all due to their husbands addictions.
Take Care of you,