How can my dad do this to his family?

I found out a couple of days ago my dad has been cheating on my mom with a coworker. I found out on his myspace page and I was extremely enraged. My mom and dad have been married for 26 years, and I think my dad got bored or something. My mom found out and they are getting a divorce. My dad said he is going to marry this other person. I am 24 years old and trying to get through medical school and I'm having a hard time dealing with this. I'm here starving with no money, living off coupons, and sneaking patients' food just to eat. But what makes the situation worse is that I found out my dad had taken $500 to sneak off with this other woman to Myrtle beach when he could have helped me or my twin sister (she is in medical school with me). My brother, who is 14 rarely sees him anymore because he's out spending time with this other women and her child. You know, I remember visiting home and asking my dad to go fishing with me, just to do something family-oriented and he always refused, saying he didn't feel like it or making an excuse. I told him off yesterday, saying I didn't want to talk to him ever again, he was a deadbeat, a streetwalker, etc. and he started laughing and then threatened to shoot himself if I ever contacted him again. I am in so much pain right now. He used to be a really good father until I was 4 years old, but then he became controlling and abusive towards my mom and the rest of the kids. But with this new girl, he's all lovey-dovey and not controlling at all, which really makes me angry. It's like we were used as punching bags and then he just goes and doesn't even try to rectify the situation and become a better person, especially seeing that he is able to control himself with this new lady. I'm just really sad right now, I've missed the past 2 days of work and just cry. I feel empty and confused... How can he do this? I mean, he did buy me things rarely and it makes me sad and guilty for accepting them. What should I do?