What I think isn't so as relevant as what you think...........your question already answered your problem. You admitted that he has addictions that he has no real desire to change. He has been given numerous opportunity's to seek help but has attempted to lie his way out of that help to you as well as the therapist. If he wanted to change, and was actively working on getting help then that would be one thing....but based on his actions he wants you to think he is getting help, while lying to his therapist and his family. Is this what you really want life to be like when you have children?
The reality is you can never know the future, but when you are granted a chance to see how present actions can reflect the future you should take it very seriously. Right now this is a relationship that has no real ties......a few years from now you may have children a home and responsibility's. Should he continue on his current path these things are going to come to a head and be a real issue in your marriage.
The truth of the matter is you can not change him, only he can make these changes and it sounds like he is not ready to. Marriage is about more then love. Its about trust, commitment, and partnership. If only one of you are investing these things eventually you will come to resent him. Which can be heartbreaking when you have children involved. Your future children deserve to have a father in their lives......should he continue and the marriage split this will be hard on the kids. Add to the fact that he would have visitation and or custody of the children even while doing drugs is scary to say the least.
As far as investment in the relationship, if you truly believe he will change then there is nothing wrong with taking it slow. Let him know that the ball is now in his court. If he is willing to change and work on this then you will consider a relationship once he has made some effort. If he fails to do so, then no real loss as you have already broken things off. If he loves you enough to change then you can go from there.
Your only 25, you do not have a big rush on the marriage game yet. Slow down, and consider your options some. Jumping into a marriage with these kinds of problems is only going to head for disaster. Give him some space so he can make his choices and you can move on in your life.
Have you ever heard the song....."Thank God for unanswered prayers"? Basically the meaning is what may seem like a true loss right now is only really a detour in your life. Someday you will meet Mr. Right (Or your boyfriend may change) and you will look back on this happy and thankful that you made the right choice. It always seems like True love when you are in love......but once you have a more clear view of the relationship it is often easier to see the flaws.
I can not tell you what is best for you..........but if you take a good long look at this you will see the truth.