Maybe Dolores subconsciously learns her way of accepting blame for everything (especially men's actions) from her mother, who here is crying and apologizing after her own husband lashes out at her.
[Ma] was searching my face for the truth. The truth, as I saw it, was that Daddy wouldn't have left if she hadn't always been Miss Doom and Gloom. (2.201)
Dolores blames her mother, and solely her mother, for her parents' divorce, seeming to forget that her dad was an abusive philanderer who holds more than his fair share of the blame.
"She didn't even cry for her own kid?" (5.96)
Dolores can't believe that Grandma didn't cry when Uncle Eddie died. She sees her as a cold, unfeeling woman, and her lack of emotion is probably partly to blame for Ma's difficulty in dealing with her own feelings.
"You and I are together in this. If you tell anyone, I'll do it. You'll probably hear the shots. Her and me will be lying up there with half our heads blown away. Two deaths, thanks to you." (7.159)
In order to keep Dolores silent after he rapes her, Jack says he'll kill his wife and himself if she tells anyone. The threat ultimately doesn't work, but the guilt sticks with her. Dolores often thinks that her own actions result in bad things happening to other people. When Rita miscarries the baby, this reinforces Dolores's tragic belief.
It was regret that had mostly motivated my mother since the night Roberta walked me back across the street. (8.7)
Ma is able to at least use her guilt to spur positive change here. She feels guilty for not looking after Dolores better when she was raped by Jack, so she tries to get Dolores into college to create some positivity in her life.
"My wife died four Tuesdays ago. Cancer of the colon. We were married forty-one years. Now stop feeling sorry for yourself and lose some of that pork of yours." (8.69)
The doctor doesn't have the best bedside manner (he makes Dr. House look warm and fuzzy), but he has a point: Dolores is using food to deal with her guilt. Everyone goes through tough times, and now Dolores needs to pull herself together, or the tough times will win.
I killed babies, mothers. I deserved this pain, was owed my misery. (9.44)
Whoa, Dolores is throwing quite the pity party for herself here. Do you ever feel sorry for her, or do you wish she'd just snap the heck out of it?
It was a pattern with me, really, I thought: Dottie's fish, Rita Speight's baby. I killed off whatever people loved. (16.26)
Here she goes again, seven chapters later, still feeling guilty for killing off things she didn't kill off. Well, except the fish—she did that on purpose, and she should feel guilty for it, although her guilt won't bring them back.
I decided to tell Dante everything that night: my parents, Jack Speight, Kippy's letters, Dr. Shaw. (21.96)
The guilt of starting her entire relationship on a lie really starts to weigh on Dolores—no, that isn't a fat joke—and she feels the need to purge herself of it (by confessing everything to Dante.
If [Roberta] had died, it would have been me who'd killed her. […] As the Buchbinders stood there waiting for their explanation, a lie about terminal illness—a brain tumor growing inside my head—created itself. (26.60)
Guilt is like a tumor for Dolores, so it's no surprise that she starts to lie about having an actual tumor.