Bit of al long one but I have ended two long standing friendships with friends that I saw as sisters. It's part liberating but also sad and uncomfortable as you sort of mourn the loss of them at the same time as knowing it's the right decision. It's a weird feeling.
One was because my friend had changed massively (as we all do over the years) but the fun, kind person I had known since school had slowly become someone who constantly b*tched about others when I was in her company, was constantly negative about everything in life and jealous of others and the bitter person she had become was draining me every time I saw her. After spending time with her, I felt angry/bitter/resentful of others myself and it took a long time for me to realise she had become very toxic influence in my life.
Things came to a head when I began doing well at work, bought my first house, got engaged and she just could not be happy for me... and instead turned her resentment onto my achievements and her b*tchiness turned on me. So I cut her off. It was really hard because of all the wonderful years of friendship we had experienced but in reality I think I had outgrown our friendship some years before but had felt the need to continue it for 'old times sake'. Still I think of her now and again and wish her well. We still have other friends in common and from what I gather, she has not changed. So I feel my decision was the right one.
The other friendship ended when I realised I was always the one offering help/organising to do nice things and making lots of effort which was never reciprocated. Again this friend could never meet me halfway with effort and it become apparent I was only needed when she wanted something. I realised that when her life was going well, she never bothered with me yet the moment she had some drama she would come knocking at my door asking for help. I also realised that I seemed to be some sort of benefactor to her as I would always be the one paying for everything. It was long standing friendship that I came away wondering if we had ever been as close as I thought or had I just been used.
In both cases it's taken me a few years to fully come to terms with the fact that two people who I loved like sisters and always assumed would be a big part of my life are no longer part of it. I think whether you're the dumpee or the dumper it's a difficult thing to come to terms with, because of the memories, the feelings and the societal value we place on needing friends to get on in life.
What I have learnt, is that life goes on. We change, we evolve and we grow and everything is temporary so my only advice would be to just let it be and keep moving forward, looking to the past is not always helpful.