Working harder is not the only answer. You must work smarter too.
If you had no marketing, no good design, no unique angle, no systems set up in place - of course it would fail.
Considering your extreme and unwarranted aversion and disgust towards marketing and design, I’m 99% sure this was the case.
People are fallible. Your mentor was not the end-all-be-all for business, nor is anyone else (including myself).
Depends on how well you took it, and if you actually did it like he said to do. Very often people say they did something, but only do like 10% of what was actually said. If you did it the same way you are responding to people in this thread, where you only consider maybe 5% of advice and reject all other… then I can see why your mentor’s advice did not work.