1. If you are a US citizen, you must file a tax return and pay US tax on all your world-wide income, even if you are living outside the US. It's one of the prices you pay for the benefit of US citizenship.
2. However, if you also pay tax on the same income to a foreign country, you can use that as a deduction or credit on your US tax return, which will reduce the US tax you owe.
3. If you are overseas more than 330 days of any particular year, you can also claim a foreign earned income exclusion, meaning your foreign earned income (income earned from performing a job or service) is completely excluded from US tax. (Instead of being taxed and then claiming a credit for foreign taxes, it isn't taxed at all.) You still have to report and pay US tax on other income like prizes, lottery winnings and investments.
4. The only way to calculate how much tax you actually owe, taking these credits and exclusions into account, is to actually prepare and file returns for those years. Turbotax is only for sale going back to 2019, so you would either need to file earlier returns by hand or see a tax professional.
(4-1/2). Tax forms and instructions books for past years are available on the IRS web site. You need to use the 2017 forms and instructions for 2017, the 2018 forms and instructions for 2018, and so on.
5. To claim the credits and exclusions, you will need copies of all your foreign tax returns and foreign taxes you paid. If you didn't save copies, you should try and get copies from the taxing authorities you paid taxes to.
6. If you prepare all 6 years of returns and find you owe a considerable sum, you may want to hire a professional to try and negotiate a settlement with the IRS. You want an enrolled agent, this is an accountant who is specially permitted to practice before the IRS.
Unless you are super comfortable reading tax instruction books and filling out the forms by hand, I would suggest that you hire a tax professional.
