I have had experience with three MLM’s over the years

Amway, Cell-Tech and ACN. You are correct when you say they promote an approach that says just keep thinking and talking positive and you will be successful. All three of the companies I have been involved with promote this idea and they all mock the “losers ” that wouldn’t stick with the business.
The reason some people make it and some don’t is because, the one’s who make it big are extremely gifted in selling people on a dream. I heard one of the top people once boast that they knew this current company was going to work for them because years earlier they were selling over-priced, barely adequate water filters and made a killing at it. They craft their stories to sound like just everyday folks but they are supremely gifted sales people and their entire focus is on recruitment not selling product. My question for you is, why after 15 years on and off, with no profit to show for it, would you be willing to try once again? You’re better off getting a second, third or fifth job. The hours spent at that will be far more profitable to you.