If my mother has full custody of my children, what can I do to retain my rights back?

It depends on how your mother got custody and what you mean by custody. If she has a guardianship that you gave her that would be subject to a different standard than a guardianship that was put in place by CPS for example. Also, if you mean she fought and gained custody over the children in Family Court then that is subject to a different standard as well. Your best bet is to gather all the paperwork you have about your case and speak to a knowledgeable attorney.

The best way to eventually regain primary custody of your children would be to reestablish yourself in their lives slowly. You should begin with seeking visitation, even if it is supervised at first, and then slowly week more time with them. The court is unlikely to find that it is in the best interests of the children to suddenly switch from custody with their grandmother back to custody with their mother, so it will help you case if you take steps toward the transition and reestablish your role in their lives.