It's not onychomycosis, it's that your nail bed is separated from the nail cover, and the nails grow by themselves to protect the skin at the end of our fingers. You cut the nail too short, revealing the finger core, which is very fragile. A small action (for example, there is mud in the nail seam, you dig it, or your fingers rub forward hard) will cause finger core injury, and then shrink away from the nail cover, which is your situation. Leave some nails, at least 1mm, and don't cut them off. The same reason.