Found recent messages on his Watch — plus proof he connected to her car

My partner’s Apple Watch died and stayed off for 3 days. When I charged it, it synced back and showed recent messages with a woman he’s hidden before — he changed her name and muted the chat. She asked if he wanted more than friends, and he said yes. Timestamps prove they’re recent, and I took photos as proof.

To make it worse, yesterday he asked me to sync his phone to the car stereo. The first thing that popped up was “Her Car” listed as a recently connected device.

This isn’t the first time. Every time I catch something, he lies, twists it, or says I’m overreacting. How do I confront him so he can’t talk his way out of it this time?