What can i do to fix a crack in my upper and lower teeth from biting my nails?

Stop the habit: It is rare to actually break teeth by biting your nails. This actually happens because you are grinding the edges of the teeth together while biting your nail. Your dentist can probably repair the teeth with a composite restoration, but as long as you are still biting it will break of again. You must stop the habit if you donot want this to happen again. It is more common to damage you gums.

Restorations: Bondings, veneers or crowns done by your dentist depending on the situation.

See dentist: May not just be from nail biting. See a dentist first then work together on the nail biting...Got to stop...Para functional habit that can create a lot of problems.

Find the true cause: The true cause may astonish you... It may be a combination of nail biting, grinding, a bad bite relationship, among others... The solution could be as simple as resin bonding, or as complex as veneers... The true cause needs to be determined, and your bite equilibrated (balanced), then you can have long term success.