Shameless barely-covered self-insert - At least she's not with someone (well, not that I know but if you haven't checked then shame on you) because that's what gets you. It's not even jealousy, because you don't really care about that guy, it's... something else. Something that makes you punch damn walls every time you see them together and makes you have trouble keeping upright when you see her or smell her perfumes and you know that she's not looking good or using them for you.
No, that's jealousy alright - I've felt it's bite too many times to not recognize its description - just not directed at whoever the guy is, but rather her/you. Sort of, as you said above, 'why isn't she putting that effort in for me' type deal.
My sympathies there, it is a tough thing to move on from.
Can't really give you much advice because I'm in a nearly identical boat, but my sympathies.