"I can understand him and what he's going through and why he keeps doing this to himself."
Unless you have been addicted to drugs or alcohol, you won't be able to do these. One addict/alcoholic helping another works because we truly know what it's like and how to overcome this.
Many people have horrible childhoods, but still manage to over come them.....
Bottom line is, get yourself help via Naranon or Al Anon and hope your fiance gets help somehow, AA, counseling, therapy, Rational Recovery, AVRT, SMART, Life Ring, SOS......are a few methods.
He has to want to stop and stay stopped, he has to go out of his way to get this help and he has to be the one to do the work so he can stay stopped.
An addiction does not mean daily use, it can mean long stretches of time between uses.
And there are many ways out of this!
Take care of you and your son now.
With love, hugs, and understanding,
~SB