Your subconscious is telling you that it thinks there is a problem and you shouldn't spend time working on a project that it feels is going to fail if you continue down your current path. Spend a few days listening to what your subconscious is trying to tell you, and refute in writing all the negative things it's telling you. If you can refute everything convincingly, then your mental motivational block will resolve itself. Don't be afraid to free-associate until you get to the root of the problem. Don't worry about spending too much time overcoming this roadblock, because 1) your subconscious is telling you there is a real problem, and 2) many times it's right. Brainstorm solutions until you get your solution. I've sometimes spent many days mulling over problems until I come up with a solution, but once I do my motivation comes right back and I can't wait to get back to the project. Everybody occasionally runs into these mental blocks; it's nothing to be concerned with if you decide to systemically attack it by letting your subconscious feed you solutions until you find one that works. Just don't write it off to "lack of motivation"; there's something your subconscious is trying to tell you. Address it and your motivation will come back.