Hindsight isn't always useful...
.... unless you share it with others who can learn from your experience.
So I'll start¹ – I wish I'd known that "getting customers" has nothing to do with "having a great product"... and of the two - "getting customers" has to come first!
What's the *one* thing you wish you'd known at the start of your journey?
¹ I'll add more detail in the comments if it's helpful.