The loss on your Roth IRA is not deductible but you should not owe taxes on the loss since you contributed the money to a Roth IRA.
In amending your 2018 return put in the full amount you contribute to the Roth IRA. Then you will have a form 8606 for the amount that was overcontributed.
With the information from the 8606 form as a part of your 2019 return, it tells the program when you withdraw the money that there is no penalty due on the withdrawal.
Link for How do I report a withdrawal of an excess 2018 Roth IRA contribution
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