I think I understand your question. The meaning of "help" changes from it's usual meaning when it follows "can't/couldn't "(sometimes "can"). In these cases it means "to avoid, to prevent, to have control over". So, the meaning shade, as compared to, for example, "to avoid", is that when one uses "I can't help", he/she is putting the stress of the fact that he/she is not able to control the impulse/tendency to do something or to behave in a certain manner.

It's short for" I can't help it............."