I follow 2 easy steps. First one, I only invest what I can afford to lose. I know it's sounds like a cliche but that's the truth. This way you'll not be scared that you lost your money.
The second one is buy and forget about your long term coins. Yes, you can profit more if sell at ATH and you buy at a lower price but it's a bit of a hassle. Is this the ATH? Should I sell? Everyone fight with this questions, so it's just a less of a headache.