I quit chewing my fingernails.
Now I have to stop sticking my fingers in my mouth so I can not chew my fingernails.
It's called a nervous habit for a reason. When stuff gets on your nerves, you do your nervous habit so it won't get on your nerves. When you stop doing your nervous habit, stuff gets on your nerves, but you can't do your nervous habit, which gets on your nerves. It isn't the fingernails. It's finding something else to do.
The first day, I was pissed. At everything, everyone, everywhere. I ate a lot of chocolate. It's getting better.
The other day, someone was getting on my nerves. I stuck a finger in my mouth. I took it out. I stood up and told her, as tactfully as I could, that it was getting old and she was getting on my nerves.
...And then she stopped.