You probably do not owe as much as the notices say
When you do not file, the IRS can prepare a return for you. It uses the income reported to it by employers, banks, and brokers, gives you no deductions beyond the most basic, and often assigns the least favorable filing status. The balance that comes out of that process is usually far higher than what you actually owe.
Filing a real return for those years replaces the IRS version. For a lot of people that single step reduces the balance substantially before any negotiation begins.

