Have you thought of having fake ones put on? I've never done it but I see women who have it professionally done and it looks really nice. I'm not sure how much something like that would cost
I have the opposite problem, mine grow too fast.
I can give a little advice on the nail biting. I used to have little OCD behavior thing when I was your age,
I swayed. I couldn't stand still. If I got nervous I would start swaying back and forth. Drove everyone crazy. I decided I needed to find another outlet for my nervousness. I started rubbing my earlobe. I know that sounds really goofy but it worked for me. Every time I felt nervous I would grab my earlobe and start rubbing. It got me to stop swaying. Its easier to replace a habit then quit it completely