This is my advice and what I would do in your situation. I would tell her that although you agreed to be 'friends', that you cannot do that right now as you need time to heal from the break down of the relationship and time to move on. Once you are healed then you will consider being a friend to her. Then... basically just DISAPPEAR. You may think she may not even notice, but trust me she will. You say she doesn't seem that sad. A lot of 'dumpers' don't at first as they have come to terms with ending things way before you have. I also find that they use the one they left as en emotional crutch to help themselves move on as it's comforting for them to know that you are still around. Only when you are truly gone do they realise what is missing.
In a few months, maybe you can spring back onto the scene, more confident, improved physically maybe if you have gained weight during the relationship, a better established social circle and so forth. I think this is the only way for her to see you in a new/different light.