Why do i bite my lip until it bleeds? how do i stop this habit?

Not a dental issue: Lip biting is not dental issue but like nail biting or continually picking at a spot its a psychological on and should be addressed with a therapist to find out why and how to deal with the causes of the behavior.

Dry lips: More than likely your habit is initiated by dry lips. As the lip drys, you will feel the flaking skin and be tempted to bite and peel the loose skin off. You will likely be less inclined to do that if you use a lip balm or even Vaseline too keep your lips moist. There are good moisturizing products available but you might avoid heavily flavored ones or you could end up in the same boat.

See your dentist: Without seeing patient can not tell you for sure but 2 causes. One you may have a deep overbite and upper teeth are too far out. 2nd reason can be a mouth breathing or sleep disorder breathing, may be snoring. Dry lip due to mouth breathing can cause lip biting which also can keep your lips wet. So consult your dentist, and or MD, or ENT specialist. Look for the cause to solve the effect. Good luc