I have many moments of defeat like that. On one hand, it's nice to be asked to go out & do stuff - something I really appreciate, but on the other, if it's going to cause too much stress to be enjoyable, are you really gaining/giving anything in the process? There's a bit of a judgment call in there - sometimes it is worth dragging oneself out of the comfort zone, but other times not. Doesn't make you a bad friend when you choose not to.

You could suprise your friend by calling them & suggesting some other time/thing to do that just works a little better with your schedule/expected stress levels. Having that schedule & not wanting to mess with it too much is not necessarily a bad thing - it's a little bit of stability in an otherwise chaotic world. Your friend would understand, if they knew all your reasoning.