Okay, wait a sec, let's get clear on the players and the plot.
You are female and were in a relationship with your boyfriend, A. Then you both started a relationship with M, who it seems is also male, correct?
Were A and M involved with each other, as in a triad, or was it actually a vee where they each were only involved with you, while you were involved with both?
So, you and A and M were together for about five months when A breaks it off with both of you. You and M continued seeing each other and now M wants to be monogamous with you. But A has come back on the scene and says he is willing to try polyamory again because he "may" have feelings for you.
You love M and don't want to lose him, but want him to know you still do not want to be monogamous. You have good memories and still miss A, so you think you want to give it a shot.
HOWEVER, what's to say A won't bail again? What made him high-tail it the first time? How long were you bf and gf before you started seeing M? What sorts of discussions took place when he left, with either one of them? Was M. given the idea that now you were "his" and did you two ever discuss being mono? I guess what needs to happen is more conversation to get clear on what you and M want, and to find out how committed A will be, but I think it would behoove you to make sure that you are doing what YOU want and not just trying to people-please out of fear of losing anyone.