My friend group is falling apart and idk what to do (I’m the middle man)

Hi. This is my first time posting here. Posting on my throw away bc it’s a bit obvious on my main and idk who knows my main acct.

TLDR; two friends in a trio are breaking up and I’m left with the pieces. How do I instill better boundaries between them and possibly make another friend talk to the other more in depth?

Anyways, we all became friends once I moved to a new city. I was friends with one girl and the girl I am friends with was friends with this other girl. Let’s call the original friend C and the secondary friend M. So M is a nice girl. I felt that her and I didn’t really “click” as much as me and C, but since she was always around, I started to get to know her better. Tbf, I didn’t really like M at first, but she warmed up to me a bit. As time went on, we were inseparable and did most things together.

A lot changed this year. I (let’s call me R), got into a relationship at the beginning of this year. I didn’t really make it a big deal (because it wasn’t) and I was managing my time well. Legitimately, a month later, M got into a relationship too. No biggie but I was a bit sus only bc we later figured out they’re moving very quickly.

I have had a lack of emotional support from M probably since the beginning of the friendship and I just didn’t notice until now. She was getting more distant, and let me preface that she hardly asked me to hang out 1-on-1 but asked C to hangout consistently. Some things transpired and now C feels that M is not a good friend and is currently removing herself from that friendship. It’s awkward on my end because now M is reaching out to me and talking to me (not ab the situation itself but just in general) and it’s kind of annoying bc we just aren’t that close (yet M thinks we are besties btw).

I really don’t know what to do. I work well in trios until things like this happens 🥲 I’m also going through a lot emotionally so I just don’t know how to go about all of this. C has a right to not be friends with her but I also think that she’s just flat out avoiding her and M doesn’t understand what’s going on, so I may have to be the one who tells her and that makes me uncomfortable. I have my own woes with M, and she’s not a bad person at all, however, she does tend to only think about herself which is annoying. How should I set up better boundaries? I know I don’t need to let them both go, but it feels like this bad.