Do you think it's okay if I bought her food and she only paid me back half?
Did she pay you back when she said she would?
Nah, she was late to pay me back. I'm pissed because she gave me half the price for the food I bought.
It is good to have helped her but it is upsetting that she took so long to pay you back. But at least she paid you some back and next time you know not to lend her money.
She expects me too much and I've proved that I'm a nice guy.
You did a generous act paying for someone's food and they were wrong to not reciprocate it back fully. Let them know how much you are owed and in the future, don't lend them anything unless they fix this mistake.