just sell it when you get a profit from it,
for example you bought it at 1 btc,and where is the right time to sell it ?1.01 or around that place.
you will lose it only if you sell it lower than your order,
don't fear because of rising in the Bitcoin price,no matter what as long as Bitcoin still exist,altcoin will also exist.
and you just need a more time from before to get profit from it,there will be one day ETH will get pumped again like other coin.
or if you don't have any financial back up,then sell it right now.
because Bitcoin will rally in the coming days until next month approximately gl in youre trading