To respond to ImaginaryIllusion... yes at the time, his attitude was very much "kid in a candy store" and that's probably a big part of why it felt so bad at first... I felt ignored, left behind, and I guess already was feeling that way but was not mentioning it.
That's a HUGE part of all this...my own part... I'd gotten lazy and quit communicating my feelings and needs to him. I'd figured any love (or lovin') was better than none, and had put away my own wants and desires etc. I felt so tired from working so much, and just wanted to let things "go" for a while. Although this stuff all went unmentioned, I know he felt this from me, and it was the wrong way to go about things.
Ultimately, if I am not present in our relationship, there is no relationship. So no matter who eventually winds up sleeping with/dating/etc whom, the point has not been lost... if one of us quits "working" (I hate that word, for obvious reasons) or holding up our end of the relationship, things start to flake out. Although my tendency is to assume I'm the one being rejected, I know now that he felt left out and unloved for the past few months because of how depressed/complacent I'd become. I'll not put us into that position again!
Thanks everyone for your thoughts. Nothing like putting this all into perspective...