It is actually quite a lot like a breakup. Aside from the physical intimacy, good friendships provide a lot of the same things that good relationships do. The irony is that it is often easier to figure out when a relationship has run its course compared to a friendship, and it's also easier to bring relationships to an end. A lot of people somewhat assume that friendships can't end at all, and feel guilty about not staying in contact with people or left a friendship drift. In reality, friendships can strengthen, weaken, and go through all sorts of changes throughout your life, including coming to an end. It's like a lot of other things throughout your life, and there's nothing to feel guilty about in that respect. It can happen for all sorts of good and bad reasons.
To give an example, my strongest friendship group is a group of guys that I've known since school. There are six of us. The one who was my best men, and me his, is someone I knew in school and was friends with, but our friendship became much stronger once we left school despite going to different universities. With another one I drifted from him after school, but we reconnected when I moved to the city that he lives in, and again our friendship became much stronger after that. By contrast, one of the members of that group has really drifted significantly from the rest of us recently. He didn't attend the wedding of one of the group earlier in the year (at which me and one other were joint best men), and has completely fallen out in the last week with another of the group who was his best man for his wedding about six or seven years ago. I'm still in touch with him, but things are strained, and that's really quite distressing, given that as a group we've known each other for nearly thirty years. But it does look like for whatever reason he's taken steps to distance himself from the rest of the group.
It's completely understandable that you're almost grieving for what you've lost with this friendship, but unfortunately these things do happen as you go through life. It's the same with your other friends. It's unfortunate to say the least that your former best friend is now bad mouthing you to others, who are now taking her side, but at the same time if they're not willing to support or at least talk to you at this point, you're probably better off without them. Either way, your former friend has made it clear what her position is, so that's that. You can and should respect her wishes on it. You can and absolutely should be sad about that, and memories of what you once had will be painful, but you will eventually move on and make new friends. It is just what happens in life. And, actually, what happens with a breakup too. You'll be fine.