Most Helpful Guy
I'm sorry this is happening to you. It's things like this, that force you to grow up faster then you'd like.
You can't change your parents or what they do.
As long as they're together and providing a roof over your head, clothes on your back, and food, about the best you can do is use their bad behavior as something NOT to do in your life!
At the same time, stop making YOUR life any more difficult. Drinking and smoking don't solve anything - EVER. Neither does cutting yourself or eating poorly. What you CAN do is take care of yourself - as much as possible.
Find friends who you can vent with, and hang out with, that have GOOD life habits. If you use your mom's cheating as an excuse to misbehave, it's only going to make your life even worse. Take one day at a time. They're NOT divorced - at least not now, so go on with what you have for the time being.