Hi there! Your mum hit her bottom - she could no longer go on as before. The divorce very well may set him off the rocks completely, he may get worse, but he needs to hit his bottom in order to get better. Divorce is it for some people - not so much for the others.
There is a lot of good information for children/spouses of alcohoilcs out there. There is alanon/alateen, it will help with understanding of what you are facing. It is not you, you cannot cure it, you cannot control it, no matter how much you love the person, no matter how much they love you - they have to want to stop the addiction. You cannot do anything about it except loving and helping yourself.
Sorry to hear of all the disappointment you've had in your short life.
You can have hope for your dad. It sounds like he may not have had any "aftercare" after his rehab. Aftercare is considered essential after treatment, but many people stop short of doing their "homework".
In AA , an alcoholic is suggested to get a "sponsor" so they can call them, instead of going out for booze when they get a "craving". So that's probably the reason why your dad has failed. You might tell him so. It might also help you to tell him, to let him know that you know where he went wrong. Communicating the reality of a situation often helps an addict who is living in "denial". You can also tell him there are free copies of the Alcoholics Big Book, either audio or in written form.
If you have time, you might even listen to it. and see the similarities, so that you can tell him that you've listened to it. and believe that if he were to trust the AA program and get involved with others suffering from the same problem and getting well, it would help him.
You can't stay stuck to a drunk, but you can give them some answers while you back away and live your own life. You can tell them you belong to a Health Forum, that has Addictions sites, and there are many that have used Medhelp to get well, by reaching out and asking for support.
Of course, he would have to be computer savvy, If your dad does not have a computer, you might just buy him the book book at an Open AA meeting and give it to him as a gift.
Do you think you might do these things for your dad?
I know this is an old post but it reminds me of my dad. I'm an only child too, and my dad and mom divorced when I was 4, and then my dad and my stepmom divorced when I was 10. He's been single ever since. He had a dog for a while, but the dog passed away last year, and he refuses to get another companion. He lives alone, just blocks away from me, but never comes to visit. Hardly ever calls. I would go to his house, but I'm getting ready to have my first child in less than a month, and his house has been neglected. On the outside, shudders are fallen, weeds overgrown, lawn neglected. Looks like a drug house in a good neighborhood, sticks out like a sore thumb, it's embarrassing. On the inside of his house, needless to say he doesn't clean, dried food on countertops, walls, unvacuumed carpets, torn wallpaper, on the steps to the basement there are drops of blood from when he has fallen :( the fridge has a leak and is molding, I don't know what to do either. I just know I'm starting my own family with a good man who works hard for me and his daughter-to-be and I can't let my dad bring me down. It hurts though, when my dad and I do talk at family gatherings or the rare phone call, I say I love you dad, and he just says "uh-huh." I think sometimes you just have to let these things go, you can't force someone to be happy and stop drinking, no matter how hard you try. Once when I was about 16 and stopped going for visitation to his house, my dad was drunk and he told me he's purposely drinking himself to death. As his only child, that hurt very very much. Anyway I just wanted you to know you're not alone. Be the best you can be.
I just want to add that my dad chose alcohol over his family. He refuses help, always has, and isn't bothered by our deteriorated relationship. The only time he tries to reach out to me is to shove political disagreements in my face, as we sit on opposite sides of the table, so to speak, in that regard. He's addicted not only to alcohol but also to confrontation and arguing. It's not a healthy way to be or to be around. I love my dad for being my dad, who he has no choice in being, but I do not love who he chooses to be, an angry, disheveled, bitter, alcoholic recluse.