I'm sorry to hear that things have been so hard for you.
I don't know about you but when I was 20, thinking back, even then I wasn't drinking to have a good time like the other people around me. I already thought that I needed the alcohol and that was because, as dubs has suggested above, I was overwhelmed, anxious and depressed. Alcohol got rid of it, for a while at least. But, as you have also found I'm sure, it started getting me into trouble as I was sick for work and losing my grip.
Reading your story, I can see a lot of how my drinking career got itself started at about your age (I'm 32 now). It got that once I started that first drink, I couldn't stop but it took me a while to realise that.
What I've learnt now is that there is no knowing where that first drink will take me. I might be lucky and not fare to bad or, I might end up dead. But one thing's for sure, I have no control over it.
Once, I ran out in the middle of the road to try & kill myself. It was totally irrational but I had no control. I thought things would never get better. But, they can if you believe. I would stress that depression was all a part of it and of course, throughout the drinking years, the whole cycle of depression-drinking-worse depression-worse drinking got worse.
I hope you're not suffering too much with your hangover today and good luck with your fight & take heart in that we are all here to support you.