Over the years I've done many long running projects. I don't think I was ever at the point of giving up, but there were times when I felt completely exhausted. The simple and straightforward solution is always this: just rest and don't try to push yourself harder. Every person has their limits, and it's better to respect them when you're doing something complex and time consuming. Proper work/rest balance is the key to achieving goals and meeting deadlines. Pushing limits is only valid at critical moments which (must!) happen rarely.