What’s one thing you wish you knew before launching? I Will Not Promote

Hi - I’m launching ~~REDACTED, I WILL NOT PROMOTE~~ soon™. It’s my first real launch *(unless you count a small free site I made for WoW tooltips)*, and I’d love to hear from those of you who’ve done a launch from 0 before. I know I should have done more pre-launch, I've learned that for the next time. But now, this is where we're at. What is just a pitfall to avoid spending time and energy on *(I've heard paid ads is a waste of time and money, does that hold true?)*, and what are small but powerful things I can do to get the product in front of potential user? Any advice - whether about marketing, positioning, community building, or just about keeping my sanity -would mean a lot. Stories of great and not-so-great launches would also be a great addition in this thread! Thanks in advance! 🙌