I have this bad habit of licking the back of my teeth, i do it so much my tongue becomes sensitive and numb? why can't i stop? what's causing this?

Suggestion: As a dentist, i would think the problem is not dental in origin, unless something specific is uncomfortable to your tongue. Most habits can be changed with a bit of effort. Behavior therapy techniques are particularly helpful in treating these habit-like disorders. I would start by asking your physician to recommend someone to help you overcome this habit if you cannot do it on your own.

Shape of teeth: Often if you have spaces between your lower anterior teeth, chips, or tartar accumalation in that area your tongue will migrate to the area. Generally, you are probably an anxious person. Your mind tells your tongue to lick the back of your teeth unconsciously to the point that you are abbessed by it. I recommend when you catch youself try feeling what the tongue is focusing on and get it fixed.