How to claim back ESPP capital gain tax paid based on FMV a lot higher than actual sale price?

I mistakenly sold ESPP shares 5 days before holding them for 2 years. I know it sucks. Purchase price was say $10 and FMV on the day of purchase was $30 but I actually sold it at $15. Last year I paid taxes for the $20 "theoretical gain" based on FMV. This year, how do I claim back the taxes for the $15 I did not really gain? Long-term / short-term capital loss at $3,000 at year?