Will nitrofurantoin antibiotics help with a absess?

First, you should not have any leftover antibiotics lying around your house! The only way to CURE infection is to take all of the antibiotics as prescribed, or you just kill off the wimpy stuff and leave the infection behind.

Second, nitrofurantoin is for urinary problems, which probably explains that nagging urge to urinate and the burning you've been having because you didn't take the medication correctly, right?