The short answer
This is the real game, and it's harder than the math. A few things that help founders sit still: write your plan down when you're calm, including your target mix and why, so your future panicked self has instructions to follow instead of a decision to make. Automate contributions so investing happens without you watching prices. Check your portfolio less often, daily glances mostly generate anxiety, not better decisions. And remember that big drops are a normal feature of long-term investing, not a signal that something broke; the returns come partly from being willing to hold through them, though of course they're never guaranteed. It can also help to notice you already stomach enormous risk in your startup, your liquid money doesn't need to prove anything. If you keep overriding your own plan, that's a sign the plan is too aggressive for your temperament, not that you need to watch more closely.
A curated summary to orient you, not advice. The resources below are the real value.