Poor you Greg, I can feel your pain in your post. I'm sorry you are hurting
Two things I wonder... is it just the label that made a change for you? If they were to stop calling it poly love and a poly relationship or what ever they call it, then would you feel better?
also, is it because you don't feel a part of it now that you have a vee arrangement with her being the hinge and having two men in her life? It sound like it was originally about sex between the two of you and now it's them and you... was there a change there?
Perhaps figuring out specifics would help...
*how much time does she spend with him?
*when you spend time together do you do special things together?
*have you organized your time so that it is balanced and dealt out in such a way that you all are happy?
*are there things she is not doing or saying that make you feel loved and appreciated?
*have you been communicating your needs and expressing your needs in a respectful way and engaging her in talking about her own needs and feelings
There is a lot on this forum that is worth reading that could help. Perhaps looking at the stickies (poly lessons learned... perhaps?), and doing some tag searchs would help.
That being said, Poly isn't for everyone and I'm not offended if you don't think it's the best time ever... personally anyways. I just think that after 10 years there is a lot of water under the bridge and you might just have a good foundation that would work... maybe some tweaking is all that is needed to get you on track to feeling confident that things will be okay.