So sad to read that you are going through this. I've followed your story and had hoped that things were going better between all of you.
The situation you are in now absolutely sucks.
It has always sounded to me like you and your husband care deeply for each other. You have been willing to shift the relationship with your boyfriend numerous times to try and help your husband feel more comfortable. Your husband has been willing to agree to a situation that he has been clear is upsetting for him. Your boyfriend has dealt with the resulting shifts in his relationship with you without complaint.
The love and effort from all of you shines through in your posts
But - maybe there is just no way to make this work as you want it to?
I reckon you've all given it a good, long try and have made numerous attempts at shifting and changing arrangements to see if they will work.
Maybe you have to choose something that isn't exactly what any of you want.
You could carry on as you are right now, allow your husband to be uncomfortable. Nobody's life is perfect and maybe having an ongoing niggle that you have a boyfriend is something that you and he can live with?
You could work to just be friends with your boyfriend and allow yourself to be uncomfortable. As before, nobody's life is perfect and maybe having an ongoing niggle that you don't have a boyfriend is something that you and your husband can live with?
You could work to be just friends with your husband and to co-parent your children while both being free to pursue whatever lifestyle makes you each happiest. It might not be the most comfortable situation but nobody's life is perfect and maybe the ongoing niggle of being friends and co-parents is something that you can live with?
I wish you well and continue to hope that you can all be happy.