Sold an ETF during a drawdown — how do you stop yourself from panic selling?

I sold an ETF position during a drawdown because I couldn’t stand watching it fall anymore. A few weeks later, the market recovered. It wasn’t a strategy problem — it was an emotional decision. For those of you investing long-term in ETFs: how do you personally stop yourself from panic selling during drawdowns? Do you have any rules, mental tricks, or processes that actually work?