Setup a hidden camera in your bedroom, film her ...... with her ex, then tell her then are welcome to each other and leave, from what you said she had zero interest in your pleasure and 100% interest in hers. You can watch the video all you want then to make you horny, but don't have to work your ass off to make money she's spending on keeping her ex happy with sexy clothes toys and likely dates too outside of sex.
Cuckolding should be pleasure for everyone, she said no to that so what's she's doing is cheating and at any point could just leave you for him at a whim.
Being submissive in the bedroom is one thing, but that doesn't mean you have to let her walk all over your life too.
Best thing to do is get them on film so you can enjoy the thrill of seeing it, then have a good hard wank and when that's done, make a sensible decision to leave her without all the emotion and confusion being horny brings into your head.
Men always make better decisions when they are not horny, in my opinion anyway.
Hope that helps