He starts to feel old in his current relationship.
Finds an "in between days" girl to keep things fresh.
Girlfriend finds out and leaves.
He tries to convince her to come back and accept the triangle, but ends up admitting that he'd rather stay with the new girl:
"and I know I was wrong when I said it was true that it couldn't be me and be her in between, without you".
In other words, he sort of feels like a dick (he is) and regrets hurting his girlfriend, but mostly feels sorry for himself for not getting away with cheating.
He starts to feel old in his current relationship.
Finds an "in between days" girl to keep things fresh.
Girlfriend finds out and leaves.
He tries to convince her to come back and accept the triangle, but ends up admitting that he'd rather stay with the new girl:
"and I know I was wrong when I said it was true that it couldn't be me and be her in between, without you".
In other words, he sort of feels like a dick (he is) and regrets hurting his girlfriend, but mostly feels sorry for himself for not getting away with cheating.