Oh Angels.... *big hugs* ....I so TOTALLY feel your pain.
In my relationship i'm the poly with a mono partner. Even tho i'm poly I too suffer from Comparisons Disease....INTENSELY. It's something i've struggled with my whole life, and it doesn't just rear it's head in romantic involvements, but in all parts of my life. As others have pointed out it is part and parcel with jealousy. I think you have to embrace that to really get past it.
After many years of pretending I was okay and telling myself over and over ..."I'm okay". I realized I wasn't. Not in the least. And I realized that only by THOUGHTFULLY WORKING on this could I get past it. This was not about anyone else or my relationship with them, it's about ME!
I was extremely afraid of going to a therapist but I decided I needed to go and I was willing to work at finding one that I clicked with. Not every therapist is a good therapist for you. I wanted someone who was aligned with my morals and beliefs and how I self-identify. This is a chance for you to embrace fully who you ARE and be totally yourself with someone who is there to support you unconditionally.
If you really want to move thru/past this you need to be willing to work at it. It's not going to be easy and it won't be quick. One thing i've learned which really hit me....(what HITS you may be different), is that all of my negative emotions are turned inward. Whenever i'm stressed or ill, or feeling blue or tired...there's a voice that pops up pretty quickly which critiques my every action and emotion. Ignoring the voice doesn't help a bit...i've learned to recognize it and then say to myself 'hey that's great but not for me!" Don't give it power by fighting it...just move forward and wave goodbye.
By happenstance through these forums I came across some really well written articles on polyamory and relationships.
I can't recommend that you read these enough. Read them all once, then read them again...and then again. There are some amazingly down to earth and powerful suggestions here for personal growth.
And as others have said, NEVER hesitate to talk to your partner.