I'm so angry at my dad

I can't even look at him right now. He promised he'd be there for my graduation, the first one in our whole family to graduate college. He SWORE he wouldn't miss it. Then, last week, he calls me up saying he has to go on some stupid business trip. Business trip! As if that's more important than seeing me walk across that stage.

Mom says I need to be understanding, that he's providing for us. But I worked my butt off for four years, juggling work and classes. This was supposed to be our moment, a celebration for all of us. Now it's just... tainted. I don't know how to forgive him. It feels like he doesn't even care.