You can go back to the first year you had losses. If you used TurboTax, the capital loss would have carried over from year to year. Unfortunately, you can't pick and choose which future tax year(s) you wish to apply your carryover to. Carryovers from this year's return must be applied to next year's return.
Carryover losses on your investments are first used to offset the current year capital gains if any. You can deduct up to $3,000 in capital losses ($1,500 if you're married filing separately). Losses beyond that amount can be deducted on future returns as a capital loss carryover until the loss is all used up.
For example, if your net capital loss in 2018 was $7,000 and you're filing as single, if you have no capital gains for 2019 and 2020, you can deduct $3,000 of the loss on your 2018 return, $3,000 on your 2019 return, and the remaining $1,000 on your 2020 return.
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