I think perhaps he's saying that he's found this amazing person who "only some dumb idiot" would let go, but he's afraid he's going to be that idiot.
"But if I'm one thing
Then that's the one thing
I should know"
Seems to me to not necessarily mean he's PLANNING to let her go, but that perhaps he's stupidly let people he loved go in the past, and he's afraid of the same thing happening again.
I think perhaps he's saying that he's found this amazing person who "only some dumb idiot" would let go, but he's afraid he's going to be that idiot.
"But if I'm one thing Then that's the one thing I should know"
Seems to me to not necessarily mean he's PLANNING to let her go, but that perhaps he's stupidly let people he loved go in the past, and he's afraid of the same thing happening again.