I know what you're going through and I want to offer you hope!
I'm 43 and have been poly for quite a few years. When my husband and I started out, he dated a much younger woman and I went through what you are experiencing. It was really difficult and I had to face a LOT of demons.
However, once I started focusing on myself and on meeting people, to talk, make friends, ask about poly.. something wonderful happened. I found out that being an "older woman" (I was 37 at the time) is actually VERY alluring in the poly world!
Six years later, I have had many younger lovers and am currently dating four partners between the ages of 22 and 37. I love them all dearly and I love myself even more than I did when I was monogamously married.
I have stretch marks, a big C-section scar, and grey hairs. I also have sexual confidence, a love of life, a sense of adventure and not many hang-ups (I left them behind once I passed 40, lol).
My advice is to remember how wonderful a thing experience is, and also that joy in life is a far more powerful aphrodisiac than youth alone.
Be courageous and be yourself, and you'll be irresistible into your 90's!