Does the sin return after sincere repentance if a person curses his repentance and considers it trivial, and are there nullifiers of repentance such as cursing it or having a bad opinion of Allah?

If you suffer from waswas (obsessive whispering/thoughts), then cast them away from you. The previous repentance is not invalidated by repentance, nor does the sin of past transgressions return. However, one must repent from the sin of reviling repentance and feel remorse for it.