I think he meets a girl he really likes but soon starts wondering whether he actually does like her as much as he thought, or if he's just somehow using her to feel emotions to help him write his songs. He apologises to the girl, saying he's a mess, he's lonely and tends to get really excited about new relationships but those feelings fade out really quickly, like it's happened with her.
He's making an emphasis on it not being the first time that something like this has happened to him (the using someone without meaning harm) by saying "another heart to steal" or the whole "this is just my tendency" line.
I think he meets a girl he really likes but soon starts wondering whether he actually does like her as much as he thought, or if he's just somehow using her to feel emotions to help him write his songs. He apologises to the girl, saying he's a mess, he's lonely and tends to get really excited about new relationships but those feelings fade out really quickly, like it's happened with her.
He's making an emphasis on it not being the first time that something like this has happened to him (the using someone without meaning harm) by saying "another heart to steal" or the whole "this is just my tendency" line.