Sindris, I think we're in parallel situations. Therefore, what I've got to say is more along the lines of commiseration than advice, hope that's okay.
Here's how I work it in my own head. Envy is insecurity, right, it's fear? The fear that there isn't enough to go around, so you can't share. But there is in your case as in mine, I bet. Then the only other thing I might advise is act as though you are secure, even when you're not. They say you can sometimes convince yourself that you're not afraid--like with stage fright--and eventually the fear will diminish. Completely unsure if that's true, but it sounds reasonable. Cognative self-therapy may not be the be all and end all, but it's a straw to grasp. My mantra is "She's a part of him, and I love all of him." Helps. A little.
And please don't slather on the additional heartache of condemning yourself for hurting. I might be new to this but it strikes me that "I should feel" is a phrase that flies in the face of a lot of the values polyamory hopes to espouse. Polyamory seems more about relaxing external expectations and restrictions, and instead serving up authentic feelings, freshly and honestly. Like really good pizza. And, like really good pizza, it's too good not to share.
Good luck, hon.