Forgot to declare loss in last 2 years back ITR

Hello Experts,

I forgot to declare Long term capital gain loss of 2018-19 because I was not properly aware that loss can be carry forwarded but can I declare now in this year and set off that loss ?  

also Last year capital gain short term loss I set off last year and can I declare this year again and set off ?   How to verify if loss already set off last year and mistakenly used setoff twice?