When you encounter something that frustrates you get up and walk around. Get a glass of water and come back. If you can't get much further after that, you need to move onto the next thing and come back later or change your approach completely to get a fresh mind on the task.

Changing the angle will help you tackle more stuff when they get hard.

You are not tied to your seat. If you need to stand up do it and stretch. Go and get some water etc. Get up and do a simple chore while thinking about a task. This way you don't stop working but your body also will return the favor and be still in good shape as the day goes on.