We just want to note that this isn't necessarily true. Mary appears to have an issue with taking responsibility that manifests itself both high and sober. If you look at the second quote in this section you'll see that Mary, when high, also blames fate a lot, instead of criticizing people's actions.
Something I need terribly. I remember when I had it I was never lonely nor afraid. I can't have lost it forever, I would die if I thought that. Because then there would be no hope.
We've already spent a lot of time wondering what Mary might have lost (see her "Character Analysis," for instance), but whatever it is, it's something representative of days gone by. The whole play, Mary's been trying to wipe away the memory of the past thirty-six years, but whatever she's looking for is something that exists only in her mind. This gives Mary an awkward relationship with her own memory – she wants to erase a part of it, but another part is the most valuable thing she has in the world.
TYRONE What's that she's carrying, Edmund? EDMUND Her wedding...