Is it obligatory for someone who has fallen into tiyarah to bear witness, perform ghusl, and say: "O Allah, there is no tiyarah except Your tiyarah, and no good except Your good, and there is no god other than You," or to bear witness and say this statement, or only to say this statement?

Pessimism (tashā'um) and belief in omens (taṭayyur) can be major shirk (definition) (polytheism), expelling one from the fold of Islam, if the person believing in the omen believes that the omen itself is the true doer of harm and benefit. In this case, one re-enters Islam by pronouncing the Shahada and performing ghusl (definition) (ritual bath). It can be minor shirk if one believes that Allah is the Doer, but considers the omen to be a cause. This does not expel one from the fold of Islam; rather, it is a sin that necessitates repentance and the aforementioned supplication. As for a mere thought that crosses the mind without acting upon it, it does no harm.