What you're talking about here is as common as mud. It usually happens that the woman in an established couple has a much easier time finding partners than the man does. I feel like I've read some version of your post a hundred million times on just this forum. I'm not saying that to belittle your anguish or frustration, but just to let you know you're not alone or an oddball because of it. Not at all.
Are you trying to get out there and meet women? Have activities that you enjoy, where you can also potentially meet people? Have an online profile at OKCupid and Polymatchmaker? Not afraid to take a chance and ask women out? Looked into any poly groups in your area?
It's not a competition. And while you are thinking about all the good stuff your wife is getting from her relationships, make sure you are not approaching women with a vibe that says "where's my good stuff?" Make sure when you engage and interact with women that you are as giving as you can be, and let them know you don't just want something from them but think they would enjoy being with you for this reason and that reason.
Hang in there and don't let yourself get discouraged.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein