Dude....I can relate, nearly 100%. Check out the book "getting past your breakup" by Susan Elliot. It helped me a bunch. Not that I still don;t have my struggles, but I am moving forward.
My therapist told me that a lot of what I am feeling is due to my wrapping up my ego and self worth with her and her responses. Not sure why I picked her. Some girl I mearly spoke with the other day took it upon herself to leave her email on my windshield so it is not as if I don;t meet other women, but as nice as it was, I still for some reason want(ed) my ex. Not even sure what...an apology, an "I did care about you". Like you said we invested in them (poor choice on our part) and we want out little reward and payoff. Want that validation. Want that feeling that we are good enough (which if was the case they should have been able to see the "good life", the "healthy way" and even use our support to get there). Truth is a few times she did try to be more comitted and I was not even sure it was what I wanted.
Truth also is it doesn't matter how good we are or what we have to offer, this is internal for them. We could be God and it still would be up to them and have nothing to do with us.
I can say all that, cognitively recognize it, but I still struggle some. it's sad to me where she is in life. Angering to me what she did to me, what I allowed her to do. Still, it is what it is and nothing I am going to do or she is going to say is going to change it. I (we) need to learn from it and move on.
I have no concrete answer for you. It is tough, I also had 4 years invested so that was a lot of time I put in! Hang in there.