Sorry to hear you’re experiencing this. What upsets me about your story is that your boyfriend knows you’re upset, and would rather manipulate your perception of reality instead of finding a solution that makes you both happy.

I haven’t been in the exact same situation, but I think it’s important to consider what effect the fb has on your relationship. Ideally, you could be aware of a fb, but your life and happiness together with your boyfriend takes priority. From your story, it sounds like this fb is disruptive, and the car situation is now creating an inconvenience for you. It also sounds like your boyfriend is aware you’re unhappy about this arrangement, hence he’s telling you half truths. He could address the problem, but he’s comfortable getting off while knowing you’re upset. Unfortunately, your bf seems happy to continue this arrangement, without consequence.

You could give him an ultimatum as someone else suggested, and that could be meaningful if he doesn’t want to lose you. You could explicitly ask him to stop seeing this person, but prepare for that to backfire. Even if he agrees, he may not be able to control that desire, and continue to pursue the fb.

Good luck!