I just put in another thread, but my journey to overcome this was realizing it wasn't a competition.
T is with me because she wants to be with me. I don't have to do anything better or more often than other men in her life. I just have to be me. That's what attracted her in the first place, and that's what keeps her involved.
I try to be supportive of her other relationships, but there are times when I voice my concerns. I do preface it with, "I understand that I might be biased here, and it's completely your decision, but I'm concerned about X." That only occurs when she's stressed out though. If she seems happy, I don't comment other than to be glad her other men are making her happy.
Me: 40 straight male in a V with
Wife: Kay - mono female - married 18 years
LDR girlfriend: Susan - poly female - 3 year relationship
Stakes - very intimate friend
Ironically named Farmgirl - local interest