It sounds like you've correctly identified that just because you are hurting, that doesn't necessarily mean that anyone else has done anything wrong. That's huge, actually, congrats! Some people actually can't get that.
If you want to not feel this way, you've gotta work on your self esteem. Have you ever been to therapy? Would you consider it? Maybe find some simple techniques and give them a try, see if you can make progress. You seem like you have the capacity for insight, so I'd absolutely bet that you can.
That said, it's ok to ask for what you need. It's drama if you break down unexpectedly and start making demands or saying you're gonna break up. It's not drama if you calmly say "Hey, I care about you both tons, but I'm still having some trouble with certain things. I know it doesn't make sense, and I promise I'm working on it, but for the time being I find I have a really hard time hearing you guys have sex. Could we talk about ways to help me handle it? Here are some ideas I had, let me know what you think might work for all of us..."
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.