I'm mad that my dad couldn't pay for a trip
Why are you mad at your dad for that?
Because he always finds some excuse not to do something.
I know you're angry, but you shouldn't expect your parents to be able to do everything. We are expected to take care of things ourselves sometimes.
My dad is very selfish for putting his own needs ahead of my own. He's useless.
Expecting your dad to pay for something and being upset when he doesn't is very selfish and entitled behavior. It's important to learn to be independent and to take care of yourself. Try to be more understanding of your dad's situation so your relationship with him will improve.