Sounds like he's just immature and doesn't really know why he's jumping from girl to girl... Most likely because he can and he's interested in trying something new.
I would forget the idea that he loves you though. He struggles to even explain why he enters into relationships, so I would be very surprised if he actually has a grasp on what love is.
Because what he says will hurt you and he wants to avoid further damage he knows he has already caused you enough pain. Trust him with his honesty, he told you he's done so move on. Knowing the dirty details will not help you.
What he's 'doing' even has a name... It's called deflection. He's deflecting the conversation away from his having to answer your questions. I'm thinking his reasons are that he is more interested in this other girl and doesn't know how to explain that to you without hurting your feelings.
I know it's never easy, but try to let it go .. He's not worth fretting over. You need to find someone who loves you for who your are, and is willing to keep his commitment to you.
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