the lack of closure COULD be indeed a factor that's why you think the feeling is still there...even just a tiny bit. i think that's normal though. i know that if i heard that my ex got a gf i would get jealous (at least for awhile)and if they break up i would feel glad and just a tiny bit "evilly" satisfied. it's human nature i guess. it kinda gives us a sense of revenge especially if they're the ones that broke up with us or did wrong to us...what goes around comes around like they say. but dont u enjoy the sense of power that u have in a way, he's single n wants u back, and you're in the meantime, happy with Shawn...girl that must feel soooo great to finally be in a place where u could be like "hah, its your fault".
Did he ask u back in the same email that he said he and his gf broke up? now i know guys sometimes wanna be on the rebound, and i think it's way too early. your ex is in a weird emotional state right now and he couldnt totally be ready for a new relationship so soon after the old one.
hope i was able to help.
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"To supress our feelings only makes them stronger"--from Crouching Tiger, Hidden Dragon