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To the LW, I sympathize, because I've been through a very similar situation with a former best friend. In my case I moved back to our city after about 3 years away (time during with my former friend was very supportive and helpful; I too stayed with her when moving back, etc.) and found that the girl I'd really loved suddenly had no time for me. She could never be bothered to make a plan, when we did make one, she'd cancel with no excuse, or show up late and leave early for "other parties" etc. When we did hang out, she seemed both bored and guarded. I found it incredibly hurtful and confronted her, but she denied that things were as I perceived or that she treated me any differently/worse than she treated anyone else. Which, I knew wasn't true because we have many mutual friends who she will make plans with, etc.
I found her treatment too hurtful to continue the friendship, which I guess goes with Cary's advice to look at the reality of the present and let the past go. But. It's been very painful to me for two years now. I still dwell on it, and regret being out of touch with her, and wish I could understand WHY and what happened.
Like you, I am in a relationship and my friend is single. It has been theorized by our mutual friends that deep down she's really jealous of my happy married life and thus can't cope with spending time with me. I still can't see that as emotionally resonant considering the situation, but maybe....?
The other theory I've heard is that women in their 30s are going through a lot of huge life changes and they get very competitive and weird with each other and lots of longtime friendships bust up, especially when there are differences in marriage/kids/stay-at-home v. working, etc.
It's sad. I wish it weren't this way. I'd love to hear the perspective of the single, 30s friend-dumpees out there, and maybe from women older who have been through this period and see it more clearly in hindsight.