How do you handle the days when you feel sorry for yourself?

So, I had a shitty childhood with abusive parents. When they kicked me out I was 17 I was homeless for awhile. My mom died a couple of years ago and since then my dad floats in and out of my life whenever he feels like; he was my primary abuser.

Once in awhile, for no known reason, I start a downward spiral which leaves me feeling angry and guilty all at the same time. Then I throw my pity party.

I know, I can compare my situation to others, I can do nice things for other people, etc. None of that works.