How do I heal after being replaced?

Me and my best friend were very close. We’d spend everyday together, they’d make plans with me and we had so many special memories and events together that were unique to us. I liked them so much and for a long time that feeling was mutual. For context, in most friendships I’d had before, I would often be a third wheel that the main two just kept around. For the first time, I felt someone was actually interested in hanging out with me and sharing precious moments with me individually. This went on for a long time, where our days were a routine together that we both enjoyed. I remember them saying they didn’t want to spend their later life without me and wanted to take every next steps together as a pair.
Fast forward to the present, one of our mutual acquaintances started spending more time with us, and, from then on, everything changed. My best friend would be spending time with me but I’d always hear our acquaintance’s name. “Hey we should come here with X.” “Hey X would love doing this.” Every damn conversation became about the acquaintance that I started feeling like hangouts with me were just an excuse to plan out events with the third person. Then, our texting dropped, plans made with me only changed to involve the acquaintance and even adjusted agreed timings for that person’s convenience. In our hangouts, I’d be getting ignored and even physically isolated, where both people would choose not to sit next to me even with seats nearby. Then, came the comparisons about what the acquaintance could do that I couldn’t. And then, the final ball dropped. As I started drifting away because of the hurt, I noticed the acquaintance filling in every gap that the lack of my presence would have made in my friend’s life. In everything special we did, I had been replaced. Any events we had pre-organised, including ones that I personally suggested, the other person was there instead. Heck, even any mannerisms and behaviours I had towards my best friend were replicated by the other person. I was looking at my own life from an outsider perspective. My day to day activities, my plans, my friendship were all there, but I was the one missing from them. And that stung the most. Maybe if they both did new activities together it wouldn’t hurt as hard, but seeing another person do everything I did in that friendship - to the point that I was watching my own regular schedule play out in front of my eyes - it broke me.
The grief is still fresh and I know my friendship will never be the same again. To go from being as important in someone’s life as they were to yours, to becoming an easily replaceable object hurt like hell. I sometimes wonder if any extra effort from my side could return things to normal, but what even is normal anymore? Where would I slot back in, when all the spaces have been filled? I know when I started drifting, my friend did get angry about that because they didn’t know why I became distant, but during that same time I was actively being replaced so that any emptiness from my distance would have no impact. I knew they wouldn’t miss me, when I saw how their life was the exact same without me being there. For them nothing had changed, but for me a large happiness in my life was gone.
I miss my best friend so badly, I wish we could go back to the way it was, but I know that it’s impossible now.

I’ve been crying over this every day and I just don’t know how to heal. I have no one else to turn to, so I ask for any advice here to just stop feeling hurt.

P.S. I apologise if the lack of names makes my post confusing but I know that the mentioned people lurk on this app so I’ve tried to be as unclear as possible.