What is the way to get rid of severe scrupulosity regarding impurity (waswas al-najasa), which has led to abandoning prayer and deeming everything impure, followed by repentance and regret, along with the fear of contaminating others and the future with my fiancé?

Dismiss these whisperings [waswas], for their best cure is to pay them no mind. The default state of things is purity, and your feeling that objects around you are becoming impure is nothing but whisperings and delusions. Simply entering the latrine without shoes does not necessitate impurity, and sweat is pure. Interact with things naturally, and base your actions on the original purity of everything unless you are absolutely certain of its impurity with definitive certainty. You may adopt the most lenient opinions regarding the transference of impurity as long as you are afflicted with obsessive whisperings.