Why did my best friend of 30 years suddenly drop me?

I was a newlywed when I first met Sarah and she pursued me relentessley for the first year of our friendship. She had a boyfriend but still wanted to spend time with me. We had so much fun together and she was always so interested in my life and happy to listen to my problems and that was entirely mutual. Her relationship split up after 18 months because he was gay and she said she preferred living on her own and just meeting up with men on a more casual basis. Her mum told me she never had any boyfriends and that she was a bit of a liar and a fantasist. I thought this wasn't a particularly motherly or charming thing to say, yet over the 30 years I have found her words to be true. She is very attached to her mother, even at age 50, her mother is her whole world and every day is spent with her. She has a dad too, but she doesn't enjoy his company that much. Sarah and her mum live a seemingly idyllic life on daddy's money just going out shopping and for drives and trips to horse shows etc. I don't have the time to join in because I have to work for a living. However, Sarah stiil expects me to drive 60 miles home from work then another 45 miles to see her. She never comes to me. The last I heard from her was before Christmas last year when I had gone to visit her. She offered to come to me next time, but each week there was an excuse why she couldn't come (usually her mum) and early this year she just broke off all contact. I am completely non-plussed. There was no row, no hard feelings, I am happy and secure in my marriage, job, part time law degree so I had no problems to burden her with. If she had problems, she never said and always said she was very happy and loved her life. I am so confused and I do miss her. I think she was jealous of all my other friends and used to be so rude to them that they stopped coming round. Eventually, I had no-one but her then she dropped me. The 1st time was for 5 years. The pattern repeated and I was dropped for 3 years and now I don't know how long it will be. I keep my other friends very separate from her now - just in case. If anyone can share this experience or who has any ideas on the psychology of what went wrong, I would be delighted to hear from you. Many thanks