I'm going to break a promise with my ex to date a different girl
Why listen to your ex? She's an ex for a reson.
That girl was acting really desperate and I'm tired of it - I liked it for a little while because she wanted me very much and it felt good to be wanted.
It's not very nice to mislead people. You are typically expected to at least honor your promises.
But when I'm done with her, I might want to date another girl.
No one knows what our future holds. Date till you find someone you're comfortable with.