You had a good idea, in maybe going to al-anon. Then you can learn to focus on you , and move on with your life, and make it as good as you want to.
nothing you write to them is going to do anything positive. But you need to have help, with your feelings being out of control. If you feel suicidal you need to call a suicide hotline. they know how to help people who are so angry that they want to self harm.
Let go of them. it will only frustrate you and anger you ,because you cannot change them, no matter what you say, do or write.
please, take care of you. maybe its time to move on, and let them go to live life as they wish. It is their life. and yours is precious, do not waste it mad at someone for something that you cannot change.
I am sorry you are hurting. let go. do something healthy for yourself. look into counselling. it helps us Adult children of alcoholics. believe me.