Paul heads in to a dark place after his mother dies. That's a bit of a pun, because Mrs. Morel represents the light of Paul's life. Without her, he is plunged into darkness.
While she was alive, Paul and his mother were completely devoted to one another. It's way over the top, too. Their relationship with each other actually prevents Paul from being able to commit to any woman in his life. He is emotionally and romantically close to Clara and Miriam at different points in the novel, but he is unable to take the next step toward marriage. Paul realizes that he simply loves his mother more than he loves these other women.
When Mrs. Morel dies, though, Paul is left with an empty, hollow life. He no longer has a reason to live. He tries to fill that emptiness with meaningless relationships with women he doesn't care about. He drinks a lot, and does anything to numb the memory of his mother. Miriam comes back into his life at this point and offers him a way out. She proposes that they get married, but Paul just can't commit. Mrs. Morel, even in death, has more of his love.
Paul continues to decline and begins to contemplate suicide. In his mind, death would reunite him with his mother and end his current suffering (whoa, that's messed up). Paul doesn't commit suicide and realizes that he must attempt to live a good life and get over his mother and her death (even if only a little bit). The sad thing though is that the reader isn't sure if Paul is actually capable of doing that. Will he be able to live life to the fullest based on his decision, or will he continue to sabotage future relationships because of his love and devotion to mom?