Obviously nothing I'm about to say is 100% because I didn't actually know Maura, or her friends, or Billy, or her family. Reading about her, and how people described her and her personality, I have an idea she could have easily been the type of person to graduate to the type of woman who was accepting of the occasional slap on the face from dear old Billy. jmo.
I think he was her first long term, serious relationship. she loved him, he took care of and protected her, just like daddy. I think he was strong willed and firm with her, just like daddy, and she did what he told her to. he was the aggressor, and she was passive. She might get mad at him for things, but at the end of the day, she loved him and he took care of her. I think she really needed and wanted a protector, and she saw that in Billy. When he cheated (by some accounts, more than once), she took him back, though clearly remained angry/hurt by passive aggressively leaving that email on her bed, and by doing her own bit of cheating. but she didn't leave him. that probably never entered into her mind. it probably never would have. one part of her probably felt very, very trapped, especially with the crazy mother mixed in. (she disturbs me greatly...it's almost as if there were THREE people involved in their relationship.) another part of her felt very protected, exactly what she wanted.
I think she was having a huge inner conflict with herself, with people planning out the rest of her life for her ZOMG PICKET FENCE, and that feeling safe and good, but also knowing she would have no control over anything, and always being the "take him back" girl.
the same email she said "I love you stud" she also said something like "honestly I didn't want to talk to anyone today" which really means "I didn't want to talk to YOU". usually when people feel the need to inject the word "honestly" into a sentence, it indicates that what they are about to say is untruthful.
so she leaves out the cheating email, but she takes the necklace and the stuffed animal from Billy when she goes. and doesn't tell him anything about where she is going. maybe her plan was to make him worry, or maybe she planned on letting him know when she got there. the two behaviours combined just show how conflicted she remained about that whole entire relationship. she loved him, but she also kind of hated him, too.