In "The Chief Mourner of Marne" by G. K. Chesterton, Mr. Mallow is searching for Father Brown to consult him about very serious problem, and finds him at a friend's house.
He entered somewhat abruptly to find Father Brown sitting on the floor with a serious expression, and attempting to pin the somewhat florid hat belonging to a wax doll on to the head of a teddy bear.
Mallow felt a faint sense of incongruity; but he was far too full of his problem to put off the conversation if he could help it. He was staggering from a sort of setback in a subconscious process that had been going on for some time.
I found that can help it mean can avoid or stop himself from doing it, but the author already said that he was so full of his problem that he can't put off the conversation, so what's the good of mentioning If he could help it?