After bad habit of biting nails, I've manages to bite off layers of my thumbnail and the nail has become very painful and sensitive. And help or sugs?

Why do you bite? Stress is a common cause of nail-biting. Have you considered exploring the underlying cause of your habit?
Break the habit. The only way to let your nails grow is to let them grow. You can try using pepper juice or something similar to help, or gloves to keep them protected. The first thing to do to break the habit is to notice when you do it, and stop it as soon as you do.