There might be a way to sell and amend you 2018 return to account for the loss. This works for foreign currency only. It happend to a friend of mine who lost a lot of money on an fx trade and was able to apply the loss against previous year gains.
You will need to consult a competent accountant. Bob’s recommendation is probably the best.
Man, if I could attach this year's losses to last year's gains it'd be awesome