How does Ntozake Shange use imagery in for colored girls who have considered suicide/when the rainbow is enuf?

In Poem Four, the lady in red reads a letter she left for a lover at the end of their relationship. In very detailed imagery, she lists the ways she loved him and all the things she did to support him. She tells him it was all an experiment to see how selfish she was and how far she could go in submitting herself to someone else. She says that she discovered, as a result of this experiment, that she can't stand "not being wanted when she wanted to be wanted" and so is leaving.