Since you spent time marketing there are 2 conclusions: nobody cared, or you didn't manage to find people who would have. Learning marketing should have taught you both those things.
The classic marketing line is: find a gap in the market or an undeserved niche, understand their needs better than the competition so you can solve their needs better. Then tell them and they should be delighted, becoming your customer. Then work hard to maintain their custom.
Coders think they just need to spaff out products, but you need to build something people actually want if you want to start a business.
It's not too late. Seriously, take some marketing courses at a university.