This to me, is obviously a breakup song.
He is remembering her now that they are no longer together. He realizes he loved her, while he was maybe unsure or didn't realize it while they were still a couple.
There is a lot of truth to this song, since a lot of people don't realize they were really in love until after it is over. We miss what we no longer have. Sometimes in relationships, people take significant things for granted until it's too late to go back.
He is taking responsibility for the relationship ending with lines like:
And you let her go
Maybe one day you'll understand why
Everything you touch, oh it dies
You don't truly know happiness until you've seen the pits of despair. I thought he was singing "You only know your lover when you let her go" which also makes some sense. It seems you learn a lot about someone during the breakup period too. Sometimes it's even more telling than the relationship itself.
The song alludes to the fact the protagonist is still reeling from the breakup. It's fresh because he has an attitude that paints the relationship as better than perhaps it really was. People have a tendency to idealize the past.