Welcome to the forums.
I think we all have the ability to love many people. For example, you don't love your first child any less after you had your second child. So that means that you love does not have to diminish if you find someone else to love.
That is not to say that polyamory is for all people. Some people prefer monogamy because they can focus all their attention one one romantic relationship. So you need to look deep inside you to see what you think you can feel. Who knows? maybe you would feel comfortable being in an emotionally monogamous relationship, but swing sexually? Maybe you want to be monogamous, but be ok with him being polyamorous?
One thing to learn on a more intuitative level is that his love for someone else does not affect his love for you. We are taught differently growing up. We are told you can only really love someone truely who is your "soul mate." However, from a practical point of view, we love many times over our life.
Try to envision that his other loves could be friends with you. You don't have to be close friends or anything, but should be friendly. After all, you do have an important thing in common:you love the same man and want to see him happy, just as he wants to see the people he loves happy.
I wish you luck.