I have experienced the ending of a 10+ year friendship before, this happened about 2 years ago. I don't wanna get too personal but basically we had a disagreement which was really more of a misunderstanding and during this time my friend said some very hurtful things about my partner :/ (this is a serious long-term relationship and we live together). I forgive everything around the initial misunderstanding but I don't know that I can so easily ignore what was said about my partner so it basically became the end of our friendship.
I know the general advice is that when your best friend says your boyfriend is a jerk you should listen to your friend but she is truly the only one who has ever said anything like that about him. Other friends/family all think we are great together so it was really weird. I don't even know if she meant that stuff or it was just said "in the heat of the moment" or whatever but still...
I still love and care about my friend. I would help her out if she ever needed anything, but I don't know that we will ever be truly close again which is very sad for me, there are still so many things that I still miss about the friendship we shared.
I've had other friendships fall apart/drift apart but this was the first really long term friend I lost and it did really hurt a lot