Can you get your child back after you relinquish your rights?

Circumstances can change. If, for example, your child is in the custody of someone who has now been arrested on drug charges, you can petition the court to award custody to you. If you don't have some concrete reason to believe that your child should be in your custody, but you merely regret having relinquished your rights, then it is unlikely that the existing decision would be reversed. Another angle is that even without a legal right to custody, you might still be allowed to visit if you are on good terms with the person who does have custody. Sometimes arguments can resolve, fences can be mended, and bygones can be bygones. It is generally a good thing for children to know their parents, and you can make that argument.