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

Help. I don’t want to have to get a gum graft for my gums but it looks like my gums are shrinking more and more each passing day. I can tell because when I wear my bottom retainer you can clearly see where my gums used to be. Now when I look at them it looks as though you can now see the root of the teeth on a couple and yesterday after brushing that were a little painful and bleeding. How bad are my gums? Should I be worried? What should I do? Is it normal for them to feel sensitive