In "To Room Nineteen," why and how is Susan isolated from the world and the people around her?

In Doris Lessing's "Room Nineteen," our protagonist, Susan is isolated from the people and the world around her.

Susan's marriage to Matthew has not been a strong one.  By all outward appearances, they are a perfect couple, but things beneath the surface are not good.  The marriage is stale, and Matthew has been cheating.  Lacking the moral support of her husband is one form of isolation for Susan.  She has no close friends to confide in, and must shoulder the problem alone; it is also expected at this time that people do not publicly air their dirty laundry, so from the outside, it must continue to appear the perfect marriage: this is what society expects.

Susan is also isolated by her own actions.  Instead of demanding that her husband leave, or at least promise to be faithful, the two determine that they will forget about this "indiscretion," and pretend as if it never happened. Of course, this is fine to say, but Matthew does not remain faithful.  And Susan really has no recourse, once again trapped to accept her lot in life.

Susan is isolated because society had not raised its daughters to have a sense of self.  There were no strong female role models or leaders to pose questions to women that would challenge the male-dominated society of the United States at that time.

Susan is lost in an unhappy marriage; she does set aside time for herself in an attempt to find out who she is aside from being a wife and mother who is unnoticed and unappreciated--but in her isolation, she has no idea how to do this.  Although many women of the time period doubtlessly experienced verbal, mental, physical, or emotional abuse, there were no agencies to protect them; Susan has no way to know what she must do in order to develop her own identity, and this further isolates her.

Of course, during the 1960s (when the story is set), prior to the sexual revolution, a woman had few options outside of being a housewife, and little chance of survival if she struck out on her own, especially if she tried to take her children with her. So if Susan had chosen to leave her husband, society would have done little--if anything--to support her.

When Matthew discovers that she has taken a room where she goes up to five days a week (while the housekeeper and nanny care for the house and children), he assumes that she is also having an affair.  Unsure what to say, she admits to it, though she is seeing no one.  Matthew is relieved and suggests that both couples get together to "talk things over." Susan makes an excuse that her "lover" is out of town and goes to her rented room, number 19.

At this point, Susan is completely lost, feeling abandoned and trapped; without a sense of self, and no hope of support from the society that dictates the accepted mores of the time, she takes the only way out that she can see: she simply turns on the gas in the little room, and "goes to sleep."