One detail sleep researchers and relationship therapists both point to is worth sitting with: struggling to maintain a new nightly habit isn't the same as refusing to.
Forgetting to insert a mouth guard, or finding a chin strap uncomfortable enough to quietly stop wearing, is a common response to being asked to change an unconscious, nightly behavior indefinitely. It's genuinely difficult to maintain, for almost anyone, regardless of how much they understand the stakes.
This reframe tends to matter most for the partner who has taken on the role of reminding, tracking, and re-explaining why a treatment matters. That role, often described as feeling like managing someone else's health rather than sharing a household with them, tends to create resentment on both sides, not because either partner is doing something wrong, but because the solution itself was built around a requirement that was always going to be hard to sustain.