You might also want to consider adding another safe word; one that doesn't mean stop, just slow down. For example, use red to stop and yellow to slow.

It will give a break to the scene and let you to check in with your Master. This way, you aren't quitting or failing (not that stopping when you need to is ever a failing--it's the smart thing to do when you've reached a limit).

Go slow and communicate, communicate, communicate.