Yes, I agree with most of the posters. Your ex basically left you to go back to her ex. She now realizes that relationship isn't going to work so she's coming back to you. But, at the end of the day she made you her second choice. Maybe you've both matured....it's possible.... but it's also possible that she knows you'd easily take her back. Are you the love of her life (and she'd treat as such) or are you easy to get back with so she doesn't have to spend much (if any) time without a boyfriend?
Are you being fair to your current gf. You say she treats you well and is in it 100% for you - which is what everyone wants, ideally. But, are you in it 100% for HER??? I think you are wishing your ex to be the way you hoped it would be - perfect and easy but that is a very unlikely scenario to be honest. But, please don't waste your current gf's time if you don't see it for the long haul - that isn't fair to her.
You are going to have to weigh the pros and cons. My opinion is if you go back to your ex it will last for several months but not much longer and hold habits etc. will creep back in and you two will eventually break up.
If you stay with your current gf the only way to move forward is to stop pinning for "what might have been" with your ex. Your ex hasn't changed. You've got a great girl now who obviously is very keen on you. You will break her heart but it is better to do it now and get it over with if you can't get your ex out of your mind.....but again.....I doubt you and your ex will make it beyond a few months (well that is just my opinion).