Simply put: if you don't believe in something, you probably won't succeed in doing it. Think of all the times you've gone to a store looking to buy a product, but weren't sure which one to buy. Perhaps you were on the fence about two or three products, and looking for insight from someone who works with them all the time - like a store associate.
Occasionally we're lucky and get the associate who is very knowledgeable, and believes in the product they sell. They're able to give you the answers you need, and typically wind up selling the product because they believe in it, and it shows.
Unfortunately, there are many more times that we get the person who doesn't really care - or worse - does not believe in the product they sell. You ask, "So, iPhone or Android?", and the answer is something like "Eh, either or is fine" or "They both suck". Not a helpful answer at all - it doesn't even begin to make you lean one way or the other. Chances are customers will just walk away without buying anything because they don't see a reason to buy it (or, at least not from that store).
If this was replaced by an explanation like "I prefer the Android platform because [insert lengthy explanation here], but some like the iPhone platform because [other lengthy explanation]", it's much more likely that something will be sold because the customer is now well informed about the pros and cons of each, as well as it becoming obvious to the customer that the associate believes in the products they sell. A great example of this is when I went to purchase my phone. The associate had the very phone I was looking at himself, and even let me mess around on it for a bit. He took the time to explain the product (pros and cons), and in the end this convinced me to purchase it.
The bottom line is, no belief, no sale. When learning Japanese, it's the same principle. No belief, no fluency.
I know, I know - it sounds more than a little cliche. Allow me to break it down a bit further:
Notice how the whole cycle begins with belief. Everything is based on that.
You have to believe in the end goal to achieve it, even if you don't know how you'll get there.
When learning Japanese, you need to be convincing to yourself that you can do it. Sell yourself Japanese. Give yourself that motivation you need to continue, and the occasional pat on the back as you go. If it helps, sit down and make a list of the reasons why you think you can, and why you think you can't. Then go through the "I can't because..." reasons and rewrite them into "I can because..." reasons. After all, if you only have reasons why you can do it, then what's stopping you?
If you believe, fluency you can achieve. (All right, perhaps that was a bit too Yoda-like, but you get the point.)
Now get crackin'!