How do I proceed if I have overstayed my visa by 2 years?

Generally when a foreign national has entered the U.S. lawfully and with inspection, and then becomes married to a U.S. citizen, the couple can succeed in a marriage-based adjustment of status application notwithstanding that the foreign national spouse may have overstayed her initial visa. Of course, there are many other details that impact eligibility. It would be wise for you and your fiance to consult with an immigration attorney who, after learning all of the relevant information about each of you, could advise about immigration eligibilities, options and goals and could offer legal representation in the often quite complex application process.