This is difficult
Compersion is hard for me, I waver back and forth, but it seems to be based on my feelings of insecurity. When I fell in love with A, it helped me to understand loving more than one person. I tend to fall in the jealousy pit when I start comparing myself to others, and that's not a healthy thing to do...although it is human nature to do so. Someone will always be younger, "prettier" (what is pretty anyway), or better at some things, or have tgings that I don't. So it is useless to compare. Instead I focus on the things I know he loves about me, and remember that he is not poly because he lacks anything from me, but because he wants to add more to what we have.
I want him to find someone...but I do sometimes get scared.