How do I approach with my ex girlfriend paying back $400 she owes me?

I lent her $400 to pay off her credit card debt. Anyway we broke up about a month ago and she was supposed to pay me back but never did. I need the money as I'm working a temp job that's about to end in 2 weeks. I don't want to talk to her as I know she'll guilt trip me about asking her for the money back and probably won't end up paying me back anyway. But she owes it to me and she's the one that wronged me in the relationship (she cheated on me), the least she could do is give me the money back I LENT to her when I thought she was a good person.