Medigap coverage needed but unaffordable. Any advice?

One day at a time. Get the arthritis and depression treated. Ask your son to handle the finances from now on. File for Medicare and / or Medicaid.

Sounds like you were hoping these things would just go away. IRS and debtors never go away. Take a deep breath, say a prayer, and deal with the tax return. Deal with your debts, that may mean bankruptcy, you may need a lawyer, you do not sound like you have been able to take care of this on your own.
If you qualify for disability, you need to apply as well. I just get the sense that you didn't really want o deal with it, later got overwhelmed and now have a big problem. Remember the best way to deal with a big problem, is one step at a time.
YOu can do it. Good Luck