To backslide is to revert to a worse state. If you've spent months breaking your bad habit of biting your nails, you'll have to care not to backslide.
If you’re sliding back, you’re not going forward or even staying in the same place. That should help you remember that backsliding is a lapse in behavior or standards. If you backslide, you’re reverting to past behavior that was not good. A reformed criminal going back to crime is backsliding. When you backslide, you slip to a lower level. Backsliding is the opposite of making progress.