Why can i not get over her?

okay, so ive been hanging out with my ex lately and as usual when i she ends up in my life i have feelings for her. and when i told her about my feelings she said she really likes me too. but she cant give the committment she knows i want. i dont know why. so i call her now and if im lucky to get a hold of her when we make plans they get cancelled last munite. she doesnt return calls or call me. im making the entire effort here and she doesnt seem to want to be with me but when we are together she gives me every sign that she really likes me. what should i do??