Sorry to hear that you are having such trouble with fingernail biting! It can be a very difficult habit to break, but there is help that is available to you!
For example, many people find that a few sessions with a psychologist
can be helpful. In these session, they can help you focus on your motivations for quitting the habit and also help you identify the striggers (such as stress) that cause you to bite your fingernails. If you can identify the triggers that cause you to bite your fingernails, you can be more aware of them and that may help you suppress the habit.
Another strategy that many people use to help with fingernail biting is to paint a clear varnish on the fingers that has a very bitter taste. There are a couple of brands of this varnish that are available on the market. You could talk to your primary care doctor
about this approach, as they can recommend one of these products to you.
Finally, some people find that getting a nail manicure or using nail polish regularly helps them to avoid biting their nails by making their nails look nicer than formerly.
Again, please discuss with a psychologist for guidance. Good luck!