Part of me wonders if I should not just leave this alone. In many ways, what difference does it all make? You (we all) may never know what really went on in his head. Besides it won't change the outcome. I do hoe it might help you for the future though.
Is this the "desperate" girl from the gym you are speaking of? I remember FF said he was turned off by her. If it's her I understand your angst.
Thing is, I really have to wonder if FF was as all in as you seem to want to paint him to have been. He may have been. You were there and we were not. I just know that guys in general - especially younger ones - just are not wired that way. All of his actions seem to be of a guy who went out with someone for three months, really enjoyed it but now has moved on. That mode fits very well with his actions - including deciding overnight, not having you meet his parents, not grieving much over the breakup. It just all fits. You wanted more. I have to wonder if you saw what you wanted to in some ways. It does not make him bad unless he all out lied to you. I just don't think he did. I think you were just both in different places.
For me it fits well with some women I dated earlier in life. Several I can think of were great ladies. I look back at our time together very fondly to this day. They just were not for me long term. Would they think otherwise? Were they surprised when I ended it? A few were in talking to them years later.
G, if he does go on to date again sometime soon it will not diminish you or who you are. It won't mean he did not really like you. It will just more mean it was not to be. You really need to stop annalizing a day romanticize get it all. It's only going to make you feel worse for longer. He's not worth that.
DonH Midwest Me 56 WAW-EXW 55 Met 11/95 / Married 5/00 Bomb 6/20/05 / She Filed on 6/2/06 / Divorced on 10/9/06 4 who'd qualify as GF since D & dated about 25 women since D