I agree with gunshotglitter, it's about all the possibilities that disappear when a relationship ends. I don't think it relates to an age gap as wisemat182 said - it could just be circumstances that are pulling them apart (the 'prison kiss'.) 'Little one' and 'kettle drum' also sound like pet names that he's using to appeal to her, but ultimately he'll still end up wondering how they got to that place - how the angel of doubt undermined their relationship.
I agree with gunshotglitter, it's about all the possibilities that disappear when a relationship ends. I don't think it relates to an age gap as wisemat182 said - it could just be circumstances that are pulling them apart (the 'prison kiss'.) 'Little one' and 'kettle drum' also sound like pet names that he's using to appeal to her, but ultimately he'll still end up wondering how they got to that place - how the angel of doubt undermined their relationship.