my guess would be depression. it can last for years unresovled. You mentioned therapy, and studies show this does help. Also medication might be an option. Some medications that treat depression destroy sex drive, but others have shown to actually increase sex drive like Wellbutrin.
One other thing for me, and I don't think this is uncommon, is that having sex consistently actually increases my sex drive significantly, especially if it is with a partner I care about. I don't think you should do anything you don't want, but perhaps just try having regular sex with your new partner, even if at first you might not be into it. Your body can get into feedback loops, and changing the patterns can signal to your body to feel a different way.