I believe that personal passion is the only sustainable way to pursue any idea and / or startup in the medium and longer term.

In the short term, there's loads of other things that keep you going: momentum, money, press, great feedback, investor interest, new customers etc. What these things however do is it kickstarts your natural adrenaline making it easy to make progress.

Once that adrenaline runs out though and when you reach a low point (of the rollercoaster), you're only left with the things you naturally have. So your own ambition, drive and passion are the things that will help get you out of that low point and kickstart the adrenaline again.

And of those things, passion is the greatest motivator to do anything. If you are truly passionate about the idea or startup you are pursuing, I believe that you will do whatever it takes to succeed.

This question and the underlying issues strike to the very core of an individual's character. In hundreds of conversations and decades of looking for ways through, around or over thorny problems with business founders, at some point most of them have muttered "If I'd known what this would take I never woulda started!" I can't offer a simple answer because my experience suggests there are a large number of ways to from the delighted "Ah Ha!", through a working prototype, to a sustainable business. Past performance is the best indicator of a CEO's or management team's probability of success ... and the past performance need have nothing to do with the project at hand. A capacity for tenacity is most valuable.
But I see another question in your wording: "How do you know when to quit?" This is terribly difficult. The longer you've been working to make your idea a success, the greater your emotional and material investment, the tougher the call. Is it more money after bad? Or is it just one more push to reach the mother lode?
This is where the advisers you collected when you were thinking about starting this journey come in. Self-knowledge doesn't seem to have become anymore prevalent since Socrates was pondering upon the need. So finding folks that will share your enthusiasm but offer candid assessments of your situation and prospects is extremely useful. Long before the day you stare at the wall and wonder if you've been a massive fool, well-chosen advisers will have helped you see a little further down the road every day. With the added "peripheral vision" they provide, hurdles may be removed, sand traps avoided and, when all else fails, a new path selected.
Perseverance and constant assessment seem to work for the most successful.

Reid Hoffman, the Co-founder of LinkedIn offers up a suggestion, "Your networks are a store of distributed intelligence that can enable you." All you have to do is know who to ask and where. I authored up an article on this subject recently to a group of small business owners at http://mvb.me/s/35d923

Here's an analogy for you. If you set off on an adventure, with no map, no plan, and no idea of what you're looking for, except that you'll know it when you see it, you're likely going to end-up exhausted, tired, and likely running out of money.

This is how I read your question detail about "the first couple of months, new-newss wearing off and the need to keep yourself motivated"

It sounds as though you have been ideating on something that you got excited about, didn't do a lot of in-depth customer development to ensure there was a need for your product, and most importantly define how you will measure success. Now, you've had *some* good moments that have made you feel like there is some potential to your idea, but you're not seeing enough growth or forward-momentum to be sure.

This will happen every time if you don't do sufficient customer development and define what metric you will use to measure success.

Anytime you start an adventure without a compass or a map, you're destined to be lost. To find yourself, look through whatever data you have, and see what's the best story you can tell yourself as evidenced by the data, however statistically insignificant. If you can't believe that you can grow that one metric in your data, then you know to give-up. If you do, then focus relentlessly on doing that in the next month. If you make good progress, continue. Otherwise, head "home"

Happy to talk to you in a call and understand where you started at, what you've done to date and what I interpret from the data and experiences you've collected to date. In a 15-20 minute call, I'm sure I could offer you good "Clarity" as to where you're at.

Is your "great idea" making you money?

Yes!

. . . you're on the right path!

Follow your passion and purpose, which are related and two separate things...Passion is that thing that you absolutely love to do, and would do all day long of you could.....Purpose is related to your passion, however your purpose is bigger than you, beyond just your passion. Always ask yourself "WHY" you think you want to do X, Y, or Z....This will lead you in the right direction.