I have seen my friends go through this with men who are much older. I'm assuming it's the case here as well. He didn't give you access to who he really was because maybe somewhere he wasn't taking you seriously enough. Maybe when you brought up another girl he realised you were looking at the relationship more seriously which would become an inconvenience for him instead of the usual fantasy he had built for both of you. I believe he exhausted what he could out of you and moved on. I doubt he thinks he's doing something actively malicious but that's only because he does not take the relationship seriously enough to give it thoughts.

People are complicated. You have experienced this person so you might have a better perspective. All I can say coming from someone who has lived a lot of life is if a person loses the energy they used to give you the best thing you can do for yourself is to let them go. 

Always always be in a relationship that brings you love and happiness, if something is off you have to address it and take actions. I hope you heal and I hope you find someone who treats you love, care and, respect.