When you find yourself experiencing jealousy, take a minute to ask yourself why. Be honest with yourself, and trust your gut. You've already identified much about it in this post. You feel "lesser" (which is a threat to your self-esteem). You feel envious, which indicates you feel you may be missing out on something. You may be conflicted (you want some of that, except that you don't). All of these are issues that can be addressed, and as you look deeper into what you're experiencing, you may find more.I don't doubt B's love for me. I almost feel 'lesser' cos I don't fall in love with others myself. I actually suspect there's envy in there for me - i.e. I find it a bit unbearable to think of him loving someone else, when I'm not interested in that. I also seem to be attached to the exclusivity of our love. Don't like the feeling of that leaching out to anyone else. Sex - it's a sacred union, exclusive, which is what for me makes it sacred. Luckily he's not challenging me on that one.
How can you get him to support you in resolving the deeper issues behind the jealousy? Recognizing that the work to be done is primarily yours, because the feelings are yours. You can talk to him about how you feel, and ask him to help. For instance, as you work on the self-esteem issue, you can ask him for reassurance of your worth and value to him. You can talk to him about what you are envious of, and explore those conflicted feelings. Talk, talk and talk some more. It helps.
I'm sure there are other, wiser poly-folk on the forum who can give better advice, but there's a place to start.