What should one do who regrets taking out a bank loan with which he married and bought a car and a house, and how should he dispose of what he acquired from it?

If the loan was usurious, then you must sincerely repent by regretting it, seeking forgiveness for it, and resolving never to return to it; for usury is one of the gravest sins and has been forbidden in the Quran and Sunnah (definition). Regret is a sign of true repentance. As for purchases of a house or car made with the loan money, there is no harm in benefiting from them, because the prohibition of the loan pertains to the liability of the borrower, not to the specific asset itself. If you repent and seek forgiveness, then the repentance is valid.