What is the ruling regarding someone who repented to Allah, but before their repentance, they went to a soothsayer, and what the soothsayer told them later came true? And how can they rid themselves of thinking about this?

It is forbidden to consult sorcerers, soothsayers, and diviners. Whoever does so must repent to Allah. The fact that what the diviner foretold comes true day after day does not harm him (the diviner), for the diviner might be using general statements, or it might be something the devil overheard and then added lies to it, or the news might be about something that has already occurred.