I've been a chronic nail/cuticle - biter for eight years now. I started when I was 12, and I'm now 20. I would say it's definitely something that I do when I'm feeling anxious, which is a lot of the time. I've also noticed that I tend to do it when I'm deep in thought. I don't know if there's any chance I could stop completely, since I've been at it so long...but I'd like to try.
I had this idea several years ago that if I was able to hold a small plushie in my hands during the day (not as a "fidget" toy, but as something to squeeze/stroke in place of biting my nails)....like a Beanie Baby, or something, then my anxiety would decrease and my hands would be too occupied for me to bite/pick at my nails/cuticles. I'm reluctant to put this into action, however, bcuz all my life, I was made to feel ashamed of depending on stuffed animals for comfort - sometimes my favorite ones were even taken away from me as a punishment for bringing them around with me. But, if I'm 20 years old and an adult now, and it's something that actually helps avert self - harming behavior (yep, nail - biting and re - opening wounds, which I also do, is classified as self - harm) I can do it if I want to, and no one can say anything to me, right?
I'd like to hear your guys' thoughts on this. 
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I wish Sterling Holloway narrated my life.
"IT'S NOT FAIR!" "Life isn't fair, Calvin." "I know, but why isn't it ever unfair in MY favor?" ~ from Calvin and Hobbes