Why Can't I Let It Go?

I work with a certain someone who used to be a good friend of mine. Notice how I use past tense. We started at the company around the same time, sat near each other, and eventually struck up long conversations and went to lunch together. When I was pregnant with my first child, she orchestrated a surprise baby shower for me. She even gave me her used crib since her boys had outgrown it. During this time she was going back to school to get her bachelor's degree part-time since she never finished when she was younger. I always admired how she was able to balance it all, with two little kids, a husband, work and family life. Shortly after she finished her degree she started looking elsewhere for work. I couldn't blame her, and in fact encouraged her since her current position held no promise for a promotion of any kind. Lo and behold, she found another position and left the company. For 4 months. That's right, it wasn't all it was cracked up to be and our big wigs asked her back for another position that was now vacant. Apparently in four months she was really able to gain supervisory and advanced experience that qualified her for this position. If you can't tell, I'm being sarcastic. Nonetheless I was happy for her. That is until she actually came back and acted like she was better than me. Keep in mind that in the way of totem poles, I was above her before (though I never cared) and in her return she was maybe my equal. Maybe. Anyway, she started treating me differently. She only went to lunch with people at or "above" her, didn't ask much about my personal life and was clearly trying her hardest to claw her way to the top. I found myself avoiding her since a simple question would be met with hostility at the most or shortness at the least. Suffice it to say we have grown apart.

The situation has only worsened over time. Whenever I hear her voice I cringe and swear under my breath. If I need a certain piece of information and my boss suggests that maybe she has it, I will do everything in my power to find it elsewhere. I am currently planning a small get-together at my house and intend to invite some work-folk, but have decided to keep her off the list. After all, my free time, my house, my choice. Why do I feel guilty about it?

What is really troubling me is that I still have all of this anger. I feel betrayed, and maybe I'm stuck in that anger stage of mourning her loss as a friend. We will never recapture the friendship we once had. Heck, maybe she was playing me all along and I only fell for it. Whatever the case, I'm fine with losing her as a friend. But why can't I get rid of the anger? Why can't I just let it go?