How to Get Over a Relationship You Thought You Were Over?

So I'm facing possibly starting a new relationship (of some sort) with a new guy after breaking up with my girlfriend of four years about 2 months ago. If you're thinking, "Gee, that's a really ridiculously fast turn around rate...", well, you're probably right.

The thing is, I haven't been torn up about that break up. (I've made another forum post about it, if this story sound familiar.) I just can't tell if that's a good thing or not. It didn't devastate me but I also haven't exactly been "happy" in the wake of being a newly single woman. I've just been weirdly apathetic, and sometimes just super bitter about things. I also haven't really talked to said ex since the fall out.

But this past Wednesday I just went out on what was basically a date (even though neither of us really called it one), and on the way back to our college we had a surprise honest conversation about feelings and things: long story short, we're both like each other. This is admittedly new territory for me, and we haven't really decided where and how we want to go from there beyond saying that we would both be willing to "try something out" (which I was already going to ask to take slowly).

As thrilling as that is for me, for some reason this weekend, I've been doing nothing but think about my ex and worrying that I'm either doing something wrong, or am going to do something wrong which will end in me screwing up whatever this possible relationship is. I don't know where this sudden episode of being upset and hung up on the past is coming from, but does anyone have any suggestions to try to get over it? Or more specifically, to try to move past lingering on my ex?