Will try my best to give a decent background without outing myself.
Friend "M" and I met at infants school aged 4/5 years and were friends for 20 years. I saw friends, but we were more like sisters and I counted her and her family as my own family, and vice-versa. She had some personal problems through her late teens and very early 20s, throughout which I tried to be there for her all the time, never judgmental, always around to listen. She came to stay with me for a few weeks here and there during which time I helped her out with bits of money because she was hopeless at budgeting for herself. Generally, we were as close as friends can be and I loved her to pieces.
As we got a little older, maintaining the friendship became more emotionally draining. We are quite different people as adults; where she is very career-focused and always knew she wanted to be a high achiever, I'm more of a homebody. Although we had similar grades all the way through school and went both went to great universities, I've never had the strict focus on what I wanted to do with my life. She always said she thought I was wasting myself and "could do better". I felt insulted by that and tried to emphasise that as long as we're both happy with how our lives turn out, we can be proud of each other.
One thing she was always disappointed in me for was having 2 children in my early 20s, not to mention then splitting with their dad. She saw this as a mark of personal failure against which I would have an uphill struggle to make my life worthwhile by having a good career. She was very dismissive of the retail job I had and scoffed when I said I liked the idea of retail management. I went back to university to finish my degree but found out I was pregnant by my new-ish partner in my second year. She was furious with me when she found out and sent me a text saying I should have a termination rather than ruin my degree and also said that she didn't believe I had fallen pregnant accidentally (quite why she believed I'd deliberately get pregnant so soon in a relationship whilst studying full time, I do not know!). I never replied to that text because I was so hurt by its contents, but when I did try to contact her again a few weeks later (by email), her response was to tell me she never wanted to speak to me again. I gleamed that she was ashamed of me for having 3 children to 2 fathers at such a young age, particularly because the boyfriend had reacted very badly to the pregnancy and we were split up at the time.
That was 5 years ago and we haven't spoken since. 
She missed the birth of my daughter, seeing my sons grow up (they adored her), my wedding - we had spent our childhood daydreaming about our lives as adults, she made me promise she could be my maid of honour, etc. For my life to have moved on so far without her really hurts, and I cried a little on my wedding day about her missing it. She's also achieved lots since then and is living the life she always dreamed of (which I know from seeing her Twitter page 
).
I don't think she misses me or regrets her decision. In many ways I should be relieved to be free of what was rapidly becoming a very toxic friendship. We had grown into adults who weren't really compatible as friends and things had run their course. However, I still think about her every day and am so sad about how things ended. I wish we could clear the air but it is utterly impossible for that to happen.
So! If you've gotten this far, AIBU to still pine like this? I've gotten over boyfriends faster fgs! My heart is truly broken and even 5 years on feels raw. Please do tell me how to get a grip.