Receding gum line. Only 20 years old. Need answers please (Photo)

There are different causes for gum recession ; - aggressive / improper brushing - orthodontic treatment / prominence of the roots - smoking- occlusal / bite problems - gum disease and bone loss You will need to see a periodontist to check the sites. Take some xrays to rule out bone loss.  The sonner the receeded sites are treated, the better the results are , and the treatment will prevent further recession and bone loss.  

According to an 18-year medical study, people with gum disease were twice as likely to die from a heart attack and three times more likely to die from a stroke than people with healthy gums. Don’t wait until it’s too late- you might first lose your teeth, and then your life.
Missing teeth are not only an appearance issue. When teeth are missing, patients don’t chew sufficiently and that can result in digestive problems. Dental implants are far more than just cosmetic; they help support the shape and structure of your mouth and face. Regular flossing and dental deep cleanings can add up to six-and-a-half years to your life.

I suggest you visit your dentist and have the situation checked out. It is possible that the retainer is rubbing on them and causing recession. Also, excessive scrubbing can be a problem. You aren't scrubbing the floors you are cleaning your teeth. Less pressure could probably be used. Your toothpaste may also be contributing to the problem. Many of the additives in toothpaste can be irritating to your tissue. Look for a toothpaste that is more basic.