This song is about the fight or flight moment of the relationship, when the girl is walking out and you either go after her and try to save the relationship or let her run away and end it.
The line 'Use my heart and not my head' is stating that he'll speak and act with his heart, doing what it thinks is right despite what his brain might be telling him, ignoring all rational thought as stated in a past comment.
This song is about the fight or flight moment of the relationship, when the girl is walking out and you either go after her and try to save the relationship or let her run away and end it.
The line 'Use my heart and not my head' is stating that he'll speak and act with his heart, doing what it thinks is right despite what his brain might be telling him, ignoring all rational thought as stated in a past comment.