This guy was an affair and he's an addict. No judgement. I just think you're going after someone who isn't available both legally and emotionally. Does that make sense? Do you think if you love him enough or help him enough, that he will suddenly become the man you think he will become? Because the answer is: probably not. He has to want to change for himself in order to change. Paying for things for him (even in secret) is only going to give him the message that he can take advantage of you financially... eventually.
A book that might help you is "Women Who Love Too Much" by Robin Norwood.