How do I stop it?

Put dental wax over the sharp part on the tooth

To stop your tooth from cutting your tongue, there is really no safe method to fix this problem your self. The only good option is to see a dentist so that he can round the sharp edges of the tooth. As a temporary solution, you can get some filling material at your local drug store and apply it around the sharp areas of the tooth. But this is only a temporary solution. Pain pill and numbing gels can also be used to simply help mask the pain. The risky proposition which is NOT recommended is to use a nail file to try to smooth the sharp area. But this should not be tried at home.