This may not be easy to hear, ok? I mean it kindly.
Neither of us are leaving, so basically my choice is to accept it.
Depending on how long you have been trying?
This could be serving the
relationship continuing. At the expense of the health and well being of the
people within it.
It is one thing to be few weeks in, a few months, but several years? After a point one could throw in the towel and say "We gave it our best shot for X years. But there is no point in putting more energy into it. It's just not a runner." And then work on accepting that.
I think a (monoamorous and poly friendly) person can thrive as one of the endpoints in a "V" style arrangement or similar. They have their one sweetie that way. And the hinge person, the polyamorous person, has their poly partners
I think a person who is (monoamorous AND monogamous) would NOT do well in a polyship. Because they are going against their own grain. They have the capacity to love one sweetie. And their preferred relationship shape is 1:1. Nobody else. There is nothing wrong with that preference.
By the same token, I think a (polyamorous and monogamy friendly) person can thrive in a Closed situation if they at least get to express their poly thoughts and feelings to their mono partner without them wigging out. "Open in Mind" if you will. That can help a lot.
I do not think a (polyamorous and polygamous (whatever shape: MMM, MFM, something else) ) would do well in a Closed situation even if they got expression. Because they are going against their own grain then. There preferred shape is NOT 1:1. They want more persons. There is nothing wrong with that preference.
What is wrong is people picking incompatible partners to start, or having grown over time in different directions.
I think it is better to accept that the shape of the relationship could change to (friendship) if a (romance) shape is not working between a mono-poly couple. They could become willing to let it go and set each other free because they DO love each other. It is not kind to have a partner who one claims to love struggling for years. It is not kind to have self struggling for years -- one has to love oneself too.
Sometimes I see people banging heads on walls, trying to fly a thing that just won't fly, struggling with the stages of grief and acceptance... it's sad.

I think it is healthier when people include "disband the union" on the table of options. And while they may chose that with regrets, and sorrows?
The mono person is then free FROM poly stuff they do not want. They are then free TO seek a mono relationship that matches their relationship shape preference.
The poly person is then free FROM mono stuff they do not want. They are then free TO seek the a poly relationship that matches their relationship preferences.
I don't know if that is the case here since you don't list how long you guys have been trying this... but keep it in mind. Set a time limit. If it isn't smoothing out and working by X date, then you may have to talk about stopping the poly thing or disbanding.
When people lock each other and selves up in a union that serves neither well and prolongs suffering? That does not sound like loving behavior to me. Sometimes the most loving thing one can do is gently let go and let things change.
I have considered separation, but he really wants us to stay together, and is very committed to me.
And you could not be together as friends? And be deeply committed to each other?
Galagirl