What's the most valuable lesson you've learned while building your product?

I bit off more than I could chew without defining clear goals and realistic timelines. Now I spend 3x the amount of planning vs doing.

Don't ignore the data! Make sure you have analytics on everything - Where are your users coming from? - Who is actually staying the longest? - Are people coming back or are they only staying on your app for one day? - etc Focusing on how people are interacting with your app/page/etc will help create actionable goals that will lead you towards whatever your vision or mission is!

that you can't put things off and tackle everything at once. And also necessarily discipline. Many, like me, used to think that sitting down and thinking all day about the work plan, prioritizing and steps to completing tasks is stupid or a waste of time. But no. This is the most important thing. And after that, you must stick to the plan.

Idle interest is not real interest. The real customers are the ones who are willing to pay for your product, use it on a frequent basis, or refer your products to family and friends. If users are showing idle interest, for example, your tech is cool but I'm not sure if we can move forward now, your goal should be to figure that out as fast as you can and move on to serve or find your real customers who need what you develop. Time is the most valuable thing when you are building a startup or a product :)