Ah, I didn't state that clearly enough. M is the one that wants to be monogamous with me, A is open to trying polyamory again.
I do agree with what you guys have said, though. I don't love A more than M, I just miss what I had with him. Going back to A would mean at least another two months of misery for having lost M, and I'm not willing to do that.
How can I show M that polyamory is worth another shot? He's afraid of me not being "his" any more. I want to express to him how I feel like I can "belong" to more than one person.