OK I wrote another post about my situation....which is not good right now. So I am going to have to keep taking the advice I give you!!! There was a time when my own happiness was completely tied to what was happening with my son just like you describe.
Even if right now I am upset and worried, that is no longer true. So what helped besides finding some good support both in this forum and through alanon?
First it was coming to a decision that I was not going to let my son and his choices, no matter what happened, ruin my life. Two he is not living with us, I think it is so much harder to detach if they are living with you and you see their moods and what is going on day to day. Three.... Finding things in my life that interest me and make me happy and doing those things. I learned how to knit, I started playing a lot of bridge and through that met some new people... And we share an interest that has nothing to do with kids. Therapy helped... And working on my marriage so that we are in this together and neither one of us is alone.
TL