Hi and welcome,
I don't see a lot of hope. More and more you just want to escape.
What coping skills do you know. What have you tried so far?
Why not offer to trade places with the bf ....he/ they can support her and you can rebuild that lost sexual relationship. I don't get why this isn't a more popular solution. Very few have tried it.
What are you looking for in a relationship now that you got a taste of poly living? Think Long term ...short term? Is this relationship worth putting yourself through extensive reprograming, behavioral modification techniques, etc, etc.
What would it look like in your mind if things ran perfect in some poly dynamic in terms of time, attention, energy, money? Ask her the same question. See if they are remotely similar. Then see if any of the specific factor run counter to each other or are likely or even possible to obtain. Maybe work backwards from there. And discuss how far from perfect are you both willing to accept. 60% ...75%...40% very hard to quantify but everyone has a tipping point and I guess conversally a settling point.