What do I do if I notice past several tax returns have not been correct?
No a good spot to be in, but fortunately you realized the mistake before the IRS (and your state and local taxing authorities) did.
Definitely consult with a qualified tax lawyer who will make sure your tax returns are corrected and deal with the IRS if payment is due.
The IRS does negotiate. This means that you may not actually have to pay all the tax, penalties, and interest legally due. For example, if you owe $50,000, the IRS may settle for something less than that.
It's impossible comment on your individual case; a lawyer with all the facts would need to do that.