Is a person afflicted with obsessive-compulsive disorder (OCD)—especially the OCD of apostasy—excused by ignorance if she describes herself as a Jewess while not knowing that this constitutes apostasy, even though she knows that describing a Muslim as an unbeliever is forbidden?

It is clear that you are afflicted with waswasa (whispers/obsessive thoughts), and its cure is complete disregard for it. One afflicted with waswasa is considered to be under duress, so they are not held accountable for statements they utter due to these obsessive thoughts. Therefore, you do not commit kufr (disbelief) by that. You must guard your tongue, for "whoever believes in Allah and the Last Day should speak good or remain silent." Imagination and self-talk do not have an effect unless you act upon them or speak them aloud. The default is that a Muslim remains Muslim, and they do not exit Islam except by certainty. Whoever does something that could be ridda (apostasy) or something else, does not become a disbeliever by that. Increase your recitation of "La ilaha illallah" (There is no god but Allah) whenever you encounter a forbidden utterance, and seek refuge from shirk (polytheism) by saying: "Allahumma inni a'udhu bika an ushrika bika wa ana a'lam, wa astaghfiruka lima la a'lam" (O Allah, I seek refuge in You from associating partners with You knowingly, and I seek Your forgiveness for what I do unknowingly).