Learning to cut losses

I’ve been DCA’ing into ETFs like QQQ and VTI for the last couple of years, but earlier on I bought some individual stocks (NIO, a few others) that are now down 70–90%. I also put about $2k into ARKK and it’s down \~40%. Overall, around $5k is tied up in these losing positions. I don’t really expect them to recover, but I’m not sure when or how to exit. I’ve held onto my ETFs since I’m still investing and want to compound. When does it make sense to sell a failing stock, and what’s the most tax-efficient way to do it? I also have a HYSA and some dividend stocks with capital gains.