Roland is one of the most complex literary characters I've ever encountered (under the flat exterior, of course). This point is driven home to me at the oddest times. I just finished Wizard and Glass, and I was just taken aback that he could share the story of Susan Delgado and her terrible fate, difficult as it was, yet balked at recounting what happened to his mother.
In my opinion, what happened to Susan was far worse than what happened to Gabrielle Deschain. If I was in Roland's shoes, that would be the story that would refuse to come out at almost all costs. However, difficult as it was, he shared it with his ka-tet fairly easily once he made up the decision to do so.
There are a couple of reasons I can think of for this ...
1. The tale of Susan serves as a warning to Eddie, Susannah, Jake, and Oy. Death is a constant for those that travel with Roland of Gilead.
2. His mother's fate tormented him more because he was actively involved.
3. Whatever else she was, Gabrielle was his mother. The nother-son relationship is complex, and Marten rubbing Roland's nose in Gabrielle's infidelity probably flamed the embers to a fire that already existed.
All of those things are probably true, but I think the crux might be even simpler: Gabrielle Deschain was a traitor. His mother, the woman who gave birth to him and sang him cradle songs, was a traitor. Her blood ran in his veins, and she was a traitor.
I think Roland was as embarrassed of what she had become as of the role he played in her death. What do you thinnk?