What should I do about the obsessive thoughts that pursue me after repentance, especially those related to my past prayers and ghusl, and my fear of seeking knowledge lest I discover mistakes I was unaware of and be forced to repeat my prayers?

The best cure for obsessive thoughts (waswas) is to turn away from them and not pay them any attention. Doubt after completing an act of worship has no effect. Seeking knowledge is an obligation upon every Muslim, and your laziness in seeking knowledge, out of fear that your previous acts of worship will be invalidated, is a deception from Satan. Rinsing the mouth and nose during ghusl (ritual bath) is sunnah according to the more preponderant opinion, and there is no obligation to repeat prayers for those who omitted them out of ignorance.