Although the IRS in some cases does use just the 1099 information reported to them, that is not the only method that can be used to reconstruct income and create a return for you.
Another method that is often used is to take all deposits to your bank accounts and consider those all to be income unless it is obvious that it is a transfer or not income (it is a bit more involved, but this is the general idea).
The disadvantage of letting the IRS prepare a substitute for return rather than filing the returns yourself is that the IRS will have little or no information on the expenses paid that will reduce your profit subject to income tax
and self employment tax.
Even the filing status, deductions
, dependents and credits that might have been available will not be included to your best advantage on the substitute for return.
If you do not file the IRS will prepare a substitute for return that will likely have much more net income and much more tax due that if you reconstruct your income and expenses and file the returns.
Indeed, it will not be easy to file these past returns; but it is much better for you to do it than to let the IRS file substitute returns.
Please ask if you need clarification or have a follow up question.