To be honest with you, and this is probably going to sound contrived, but let it soak a bit, you have to have something else more important than the sex to keep the sex good. If you are sincerely trying to balance an exclusive sexual relationship, there is no way to do it if the sex is the only exciting thing. What else are you guys doing PHYSICALLY together that makes y'all laugh, or gives you two a heightened sense of emotion? What are you two building in common outside of condom receipts?
You cannot build a relationship on money OR sex alone. If neither one of you is doing SOMETHING adventurous or just productive, that the other can be supportive about, and create some type of bond, so y'all can take a break from just smashing each other's pelvic bones , and allow some of that energy to build back up in addition to the new energy from the successes, and ironically failures, too, then the sex ain't going to get better no matter how many toys, edible thongs, or anal beads you experiment with...
Just my dime and nickel...