She can't forget him, but she doesn't know why. She feels like she might be "mad", or "proud". In reality, I think she needs (wants) to get back together with him. Or, yeah, get over him. Just two options essentially.
This song seems so simple but it is full of endless meanings too- a wealth of emotions just literally boiling under the surface. Love can be really sucky at times, and this is one of those times where you feel lucky that you are not involved in it (me, I think;)).