Is it obligatory for the questioner to perform ghusl and re-fast the past days due to the discharge of yellowish or greenish secretions resembling a thread, after she experienced inappropriate thoughts and images, given her suffering from obsessive-compulsive disorder and difficulty distinguishing between semen and other discharges?

You must ignore whisperings and pay no attention to them. If you doubt what has exited, whether it is manī (seminal fluid) or something else, then ghusl (ritual bath) is not obligatory upon you, and your prayers and fasting are valid. Do not perform ghusl unless you have certain conviction that manī, which necessitates ghusl, has exited. Yellowish and brownish discharges are not considered menstruation unless they occur during the customary period or are connected to blood.