Please take this with a grain of salt, as a possible perspective (I don't know if it's helpful or not). Let me detail the roles in the analogy:
You made a search engine. You found you weren't making any money, so you studied how other search engines made money, and you found that they had ad and paid-position results. You didn't like that, so you didn't continue. Then google took the approach of trying to give people what they want (fast, uncluttered, relevant results), and then later (literally years later) working out how to make money from it (this was a whole project in itself: text ads, relevant to search, priced by auction: adwords, seemingly "inspired" by goto/overture).
Your point 3 (motivation) seems to be the show-stopper. My suggestion is to consider if there is a way to solve this problem - not in order for you to make money, but in order to help people diet. Illustrative examples (recall that I don't actually know anything about it):
- Make it continually novel, with new content being added all the time (like HN, or WoW), or the "achievement unlocked" of some games, or Nintendo games. Or a new diet every week. Or even as a platform with new perspectives on the diet coming out each week or day (like a daily horoscope or cartoon maybe?). Don't know if this would work, but the idea is to attack the problem of novelty wearing off.
- Or expand on the solution you found in your personal note: tell people to buy smaller plates and glasses; and buy an exercise bike, with instructions about how to set it up with the TV. Maybe this seems trivial and obvious, but I'm guessing it wasn't the first thing you tried yourself - maybe encouragement and guidance would make a huge difference for some people.
Maybe it seems that you can't make money from this; but (I believe that) if a business finds a way to help people, it will find a way to make money. The thing that is potentially exciting is that maybe there is a fantastic way to be extremely helpful to people in dieting, that all the other businesses have missed, because they were focussed on the business model that worked - instead of doing that, find a better way, like Google did.
It sounds like you've done a thorough and intelligent job (and also that you are sick to death of it), and that you were excited about your solution, not about the problem. Let me emphasise that I really have no idea if there exists such a solution as I'm outlining - I just wanted to communicate a focus on your customer's problem, not business models. Reading back that previous sentence, it sounds kind of rude to me, but I hope you'll understand how I mean it.