When you feel stuck

• hunker.  Try to focus on several simple issues.  Attack them first and build up confidence. 
• do something else (but not too long).   For me, I usually spend time to work on foreign languages or just write down answers I worked out in the past in other books. 
• Think of a simple plan to implement for your problem.  Implement it and learn something about the plan and the solutions
• For the simple things you know, can you think of new variations of them?