To me, the song is talking about a guy who is dealing with a lot of depression and anxiety, maybe even dealing with some trauma. He meets someone who makes him feel good again, who makes him feel right again. I can't speak for exactly what he means by passenger, but I think he means he's not necessarily the one in control. They may be driving the car, but he's right there with them, sitting in the passenger seat. "Tell me it won't hurt" I read as, tell me you won't hurt me. Like, I don't want another relationship that's going to end badly. "The old me, won't work" is like "I don't want to be who I was. I want to be better with you" "Now I can accept what I lost. You can make me balanced, but you can't make me whole" is saying "I have holes, I have lost things but the happiness you bring balances the dark stuff. It doesn't fix the holes or fill the void, but it helps me to stay afloat."