One Direction - Where Do Broken Hearts Go? Lyrics Meaning

What I genuinely think is that this guy was messed up from the beginning and didn't know what he wanted and was was wasted until he found this girl that actually changed him for the better but due to who he originally was, which was 'a person who was reckless and made mistakes',he made thesame mistake but with something he didn't know was precious to him which was 'her', and that the guilt he felt about those past mistakes couldn't be compared to the guilt of sadness written on her face when he walked out on her.
He left her all alone in the world broken down and hurt which made her make mistakes of her own maybe. And she remembers all their fights and yells in where she had learnt to grow and actually love him but the pain and the memories kept hurting her which refers to the line 'so I built u a house from a broken home'

The guy thought hard about her because he still loved her and wanted things to work out so he thought about what she said, and all the tthings little things he didn't do and wrote a song about it which actually was hard but when he did, he knew how to make it all better again and fix up the mess he had done 'How to fix up a heart that I let down'

And so he tried finding her but it felt impossible, he'd call her but she wouldn't pick up. He knew he'd hurt her bad and broke her heart which made him wonder where she'd run to...where her heart would run to, but also hoping she wouldn't run to someone else just for a rebound because he still loved her and didnt want her making mistakes like he did