Why does it have to be so hard to fight off the urge to call him?
I think because at that time, we are quite literally addicted to that person and contacting them is what our addiction demands. Why does someone who can barely breathe suck smoke into their lungs? Why does someone with 4 DUI's continue to drink? Addictions push us to do things we know are bad for us. The best defintion I've ever heard of an addiction, and I've heard many in the course of working on my own, is that it's a physical compulsion combined with a mental obsession. So not only do you have a physical craving for it (your comment about wanting to take him to bed) but you can't stop thinking about it (this speaks for itself).
Also, check out the stuff on http://www.coping.org about codependency and detachment, there's some interesting info there. I don't fully subscribe to everything on that website but I do think there is some good insight.
Me: 32 in OH Wife: 29 in MD Married: 4 years No kids Seperated 14 months