Is it obligatory for someone suffering from obsessive-compulsive disorder that affects her prayer and thoughts to ignore the shapes that appear to her as crosses or faces, and is she sinful if her gaze falls upon them or she involuntarily feels that they are speaking to her, and what should she do to overcome these thoughts?

The whisperings (waswas) have greatly affected you, and the cure for them is complete disregard and indifference. Pray on any part of the rug, and ignore the whisperings of Satan about the presence of images of people or crosses, and proceed with your prayer. These whisperings do not take you out of Islam, nor is there any justification for repeatedly uttering the Shahadatayn (the two testimonies of faith), nor do they affect the soundness of your faith; rather, you will be rewarded for resisting them. You must continue to struggle against these whisperings and strive to get rid of them. You may consult trusted doctors or the counseling section of this website.