I think that Lessing's characterization of Susan is enlightening to both women and men.  Susan's refusal to openly discuss her husband's initial infidelity as well as her own need to get away to "Room Nineteen" is vitally important.  Lessing's point seems to be that there can be little hope for people in the modern setting in an emotional context if there is not complete transparency and total openness in their lives.  For women, in particular, Lessing's work speaks to the idea that acquiescing to social conformity at the cost of being psychologically validated and emotionally heard is not a successful plan.  Susan continually defers the reality that surrounds her.  She never confronts her husband's infidelity and how this has ruptured the fabric of their relationship.  Susan does not speak to this and, instead, finds herself internalizing both the experiences of infidelity and her own existential identity.  The ending in which Susan "drifts off into the dark river" is one in which Susan's silence is what ends up playing a role in killing her.  For modern women, the enlightenment element is that women must capture the moment in which their voice needs to be heard.  Bypassing it because of social conventions or even merely selling themselves short in the belief that their feelings are "normal" is not a recipe for happiness.  It is here where there is an enlightened feeling to the text, almost one that makes Susan a cautionary tale and a looking glass into our own lives.