What should be done if older single woman wants child but cannot conceive? Is that her lot in life?

Should, or can? There's a lot that can be done; what should be done is a deeply personal, private decision.
Why a child ? If you are stuck on the idea, find an unrelated counselor and talk it through.You can learn coping skills to deal with it, or make a decision to adopt ,foster or do other work to fulfil a personal drive. Dwelling on the lack of progress in the past can interfere with your ability to move forward in life. When you have something to offer a child, there are many forms in which to give it.