YOU need alanon/Alateen:
http://al-anon.alateen.org/?gclid=CPWLhu...fgod0l8A0A
Go find a local meeting and go to a meeting and seek their help.
They will help you to see the addiction side of things a bit more clearly, and will tell you what roads are available to you to deal with his addiction.
As for cheating and relying on this 'friend' - I call bull-shit. He needs a program of recovery, possibly even 28 days in rehab to dry out - depending on what drugs he takes.
He also needs to get out of the scene, get away from old playgrounds, play toys and play mates - or he will just be stuck in using again and again.
This whole "I can do it on my own" thing is a typical denial ploy - which is basically saying 'I don't have a problem, I can quit anytime I want'.
Mid-life crises is a mess - and he has decided to make it as complicated as possible by chasing drugs.
I fear this isn't going to end well, not unless he wakes up and sees what he is doing to you and the 'us' of your relationship.
Unfortunately, the probability is that he won't wake up until the Us is over.
You also will need therapy to deal with the betrayal, and to work through all of the other crap he has done and the crap is currently done and the crap he will continue to do until he makes changes in his life.
I can guarantee you one thing, YOU do not have to do this alone, and shouldn't. Seek help now for YOU in order to prevent the side effects that will hit sooner (than later).
I feel for you, but there is nothing more that I can do except try to get you to seek help for yourself.