It means, and this is pretty obvious.
he loves this girl, and they are in a relationship. they have been together for a really long time.
but at some point in time, he has started to feel differently. he feels alone and doesn't feel the same connection.
he has to leave her, it's not fair to either of them for them to stay together, no matter how hard breaking up may be.
but while he is trying to gain the courage to break up with her, every so often he will remember how things used to be, how in love they were.
which makes it impossible for him, but he knows he must go.
It means, and this is pretty obvious.
he loves this girl, and they are in a relationship. they have been together for a really long time.
but at some point in time, he has started to feel differently. he feels alone and doesn't feel the same connection.
he has to leave her, it's not fair to either of them for them to stay together, no matter how hard breaking up may be.
but while he is trying to gain the courage to break up with her, every so often he will remember how things used to be, how in love they were.
which makes it impossible for him, but he knows he must go.