This is a song about someone who wants to be romantically involved with someone else who isn't that interested. It could be that they are already in a relationship, or perhaps they've been building up a romantic friendship, but it's not quite got to a relationship status yet - "My heart is a stone, you don't want to go there" says she isn't interested, or maybe she's looking for something more. The heart is a stone reference can mean a variety of different things, he could be saying it's hard for him to accept people in, that it takes someone special to warm his heart up, or perhaps she just wants someone who's heart is better than a diamond, as a means to "upgrade" her preference, that he's not good enough. Him saying "I don't mind if you try" is him trying to express his feelings, that he wouldn't protest to her letting him in and seeing where things go. "No one knows how I love you" says to me that even she doesn't know, he clearly doesn't expose his feelings often, he keeps his feelings to himself. Not telling her has allowed her to become distant, maybe she thinks he's not interested but he's explaining that in fact he is, he just doesn't tell others how he feels. The outside love moves on and imaginary world reference I think are just saying they can only be together in an imaginary world, a dream world, that he's accepted that they don't belong in the real world together, that it's not going to work out, however still continues to declare his love for her.