Hi welcome back. I am sorry you are dealing with this. Raising three children and being involved with an alcoholic is tough work. Since you two aren't living together I suggest you get a child support order for your youngest. Dating or not he is legally obligated to help support your baby. Make it legal.
Also make sure you are getting all of the assistance you may be entitled to. Medicaid or CHIP. Food stamps, utility assistance, housing assistance. Free or reduced lunches at school for the older ones.
To answer your last question here is the thing. You have no choice but to accept reality. Unless he wants it. Chances are huge he won't change. If you don't accept that you are keeping yourself mentally and emotionally trapped. The reality is you are a single mom of three kids. They are your reality. Take care of them and take care of yourself.