How can my mother enjoy Heaven, knowing that her atheist son will be tortured forever in the fires of Hell?

Obviously, it is not important to you that you will be tormented forever in hellfire. Why should it bother your mother?

Your mother's existence is not all about you, nor your father's or any of your friends' (Luke 14:26). Therefore, you should stop thinking that the universe revolves around you. If your mother is a Christian, she holds God first and foremost in her heart and soul and rejoices to see Him glorified. He is glorified when He shows His mercy upon those whom He chooses to show His mercy upon, and He is also glorified when He dispenses justice to those upon whom He dispenses justice (Romans 9).

If your eternity in hell should be of concern to anyone, it is you.