I'm a huge nail biter and one thing that has helped SO much is a fidget ring. I like it more then other fidget things because it's attached to me so it's harder to lose or forget and it's descrete enough I don't get self conscious. I got mine off of Amazon but they have a bunch on Etsy too!
6mm Unisex's Stainless Steel Rose Gold Frosted Spinner Ring for Anxiety 8 https://www.amazon.com/dp/B0719V4N7C/ref=cm_sw_r_cp_apa_OQNtBbTR2FR3F