How to ensure you're not investing too wide as opposed to too deep?

How do you guys deal with the temptation to over-diversify? Or, better said, too invest too wide and not deep enough? Note, this is in reference to my individual investment account, not my roth or 401k.

I get that it's generally safer to diversify, but I know that there are explosive gains to be made when you find your NVDA, SNDK, etc. and invest some decent capital into it.

I want to be in quantum, photonics, space, software, AI, defense, etc., but I know that I'm likely leaving material gains on the table by stretching too thin.

Thoughts? Anyone else struggling with this?