Hi Verenchy, glad you found the forum, but sorry for what brings you here, of course.
First things first - you didn't Cause it, can't Control it and can't Cure it (the 3 c's).
You mentioned when you first met that he didn't drink a drop. When did that start to ramp up, was he drinking socially or getting plastered on a regular basis? I ask because he may have already been an alcoholic when you met him, he may have been in recovery and not drinking - but he certainly is now.
You ask how he can "forget" you all so quickly. He hasn't forgotten you, it sounds like he just doesn't care about anything anymore. He just wants his alcohol and his pills. You tried to come between him and his drugs of choice and he wants (needs) those drugs. You became the enemy. This is not unusual for addicts.
I hope you will take time to read the other threads in the forum and find out as much as you can about alcoholism (for you, not for him). There is also a book that is the most recommended here - Codependent no more, by Melody Beattie. You may want to pick up a copy as you might find it really useful. It has a lot of information about boundaries in relationships etc.
I take it he doesn't work? You may want to get a lawyer and start right away on filing for child support. If you can't afford a lawyer, contact your local law society as there are probably lawyers that will help you for little or no charge.
The best thing you can do for yourself? Take care of yourself and your children and try to focus less on him. I know, that's really tough, you are hurt and confused right now. But by consciously focusing your energy on yourself, you will find some relief. Do things you like, get outside for walks when you can. Eat well, get plenty of sleep, talk to friends, post here, read here.