well a lot of the most screwed up people I know really look all put together. They don't ever deal with anything, just hold it in and when it finally blows up it goes in a big way.Also any addiction doesn't help, and I've met a few people who were borderline drinking problem types that can when things start to fall apart go over to serious drinking problem pretty quickly.
So Bree falling apart the way she is doesn't seem out-of-character to me. I think the producers did say in an interview that she falls faster and harder than you see in reality, but part of that is the way the show is paced.