Unfortunately, any advice is highly sensitive to your personality and portfolio. You need to match your portfolio to your personality.
Personally, I've always been (relatively) immune to FOMO. Watching others get wrecked during dotcom (and other bubbles) makes me almost allergic to chasing "hot stocks". And my returns have probably suffered at times by avoiding great companies just because they were "hot stocks".
I also get excited during drawdowns, because that's where the best opportunities are.
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But a few specific tricks I use:
1. When the pain gets too much, I might sell a TINY part of one of my positions. This is a psychological trick. I KNOW that I should be buying. But the act of "doing something" is usually enough to satisfy the panic. Then I can either ride out the drawdown or start buying with a clear mind.
2. I always assume there will be a 50% drawdown in my portfolio (or a specific stock). This is just the cost of buying stocks. It's like a new car. The day I drive the stock off the lot, I assume 50% in depreciation. But it is worth it, given the higher returns from stocks.
3. I like Buffett's idea of "equity bonds". An easy trick is to look at the earnings yield instead of P/E. A 50% drawdown just means your yield doubled.
4. Write an annual investor letter (to yourself). This is the time to reflect on position sizing, etc. Not in the middle of a drawdown.
5. Go "window shopping" for bargains even if you don't have cash. For example, want CSU to $2000 so I can buy more. (never mind that my 20% position in a 50% drawdown).
6. I try to only buy stocks that I would feel comfortable making 100% of my portfolio. And I prefer to invest alongside founders who actually do have ~100% of their portfolio in the stock.