You have come to the right place to support you through this. It is a very supportive and intelligent forum.
There will be people who have been through what you are going through but you will find value in lots of places. Our experiences might be different but polyamory brings up a lot of the same emotions.
Like you I'm here because my partner has an emotional other (he'd like it to be more). Like you will have to, I am learning to deal with the insecurities and jealousies it brings up. I don't know when I joined but it has only been a month or two. In that time I have been able to go from wondering whether I should just cut my losses and move on, to experiencing a depth and intimacy of relationship I didn't know existed.