Simply put in STOP LOSSES that sell it off before it tanks. I set mine at 15% below my current value. Some people are lucky enough to have 'trailing' stop losses where you can set a % and it stays in effect at the stock increases. Takes away a lot of fear.
My problem is that I'm just not cut out to be a trader, and never intended to be. It's only the unexpectedly massive success of Tesla that's thrust me into this position because as long as it was in the hundreds I could just say "I'm never going to sell so the price doesn't matter." But then it's suddenly in the thousands and my brain goes "WHAAAAAA???" and has a melt-down.