I have a black line on my nail that has showed up about a month ago. i'm not sure what it is. it was not caused by any injury. ?

Bruise, injury most: likely. However, without a high quality photo, guessing accuracy is greatly decreased. Suggest use the phone system; AKA internet & a search engine, e.g. https://www.google.com/search?q=nail+bed+black+line select images & look for a similar image. Be aware, depression, ADHD, etc. (like all diagnoses) = names for patterns of behaviors, not insight into reasons or effective/helpful treatments.