When I loose all motivation I take a week off. Generally within a few days I can't wait to get back to work. But I only dabble. I spend a few hours thinking or researching a small, stimulating area of the project. After 7 days I am fully charged up. I've seen my friends. I've spoken about the project, I've got new ideas and I've had a rest.

If after a week I still can't face it... well I'm probably screwed. The idea may be fine but I've lost it. I generally soldier on for a month, take another week off and repeat. If the cycle lasts a few months and I still can't get past the pain. I think (for me at least) it's time to move on.

I can't really answer the specific questions you pose because, well if I could answer them, I'd be VC boss dude with a suit and a very fast car.

Time away from most problems seems to make them less of a problem in my experience.

good luck