I also think this is about a lost chance / missed opportunity that the singer regrets.
He was either in love someone and couldn't see that she liked him too. Or, he wasn't in love but when he found out later that she liked him and realized that maybe she was the one, it was too late.
Either way, the singer missed his chance and now this girl is stuck in his mind.
"Now I'm stuck out of line" basically means that it would be out of line for him to try to do anything with this girl now. She's gone and he needs to move on but can't.
I also think this is about a lost chance / missed opportunity that the singer regrets.
He was either in love someone and couldn't see that she liked him too. Or, he wasn't in love but when he found out later that she liked him and realized that maybe she was the one, it was too late.
Either way, the singer missed his chance and now this girl is stuck in his mind.
"Now I'm stuck out of line" basically means that it would be out of line for him to try to do anything with this girl now. She's gone and he needs to move on but can't.