This is a very common sentiment and is echoed almost everywhere startup related. You can read PG's essays on the matter or watch some Startup School videos.
I understand you may already have a product so hopefully others can give you more advice.
I think this popular advice is flawed. It's survivorship bias in advice form.
If you happen to have a good profitable problem you go solve it and become successful and then turn around and say "Look! I solved my own problem".
There exist many people whose own problems are terrible for turning into a product for many reasons. They would be better off if they attempted a more practical problem that was not necessarily their own.
I also see founders reverse-engineering their story to mask the fact that they just went after a feasible profitable idea.
For example they create a customer support software (as an example of a crowded competitive field).
But because it's not sexy to not have a dreamy story they manufacture some forced story line like "We were running this small consultancy company and needed customer support software. After trying tens of existing solutions we just weren't happy with their X,Y,Z so we decided to solve our own problem for good!".
Edit 2: Addendum to above: Even with the suggestion you've made, that assumes you know the problem BEFORE you have a product. OP has a product.
But yes. OP, if you combine both of these 2 cents, your 4 cents would be: Know the problem before you have a product not the other way around :)
Edit 1: Also, note that there are new set of biases to worry about in the approach you have pointed out; does not make it invalid, but you also have to be careful there.
First you find a problem, then solve it. Go read everything on https://stackingthebricks.com/
(I've been there. Built a product. Talked to potential users to try to find a problem it would solve, they talked about their problems. We ignored those problems, focused on our technology instead. Far far better to find the problem first, and then build a product.)
He's asking how people found the problem they ended up working on.