Ok, 1st you are your best barometer, you know yourself, so trust yourself. 

2nd, so far in life I have found truth in the cĺiche that you regret the things you don't do far more than the things you do. 

3rd This is why we have gastro / food poisoning. Yes and plan on time to recover, plan set time after where you fall apart and just stop being a responsible adult for a while, curl up in a ball and do whatever it is you do to take care of you. If you wake up the day of and just can't face it no matter what, well damn, you got gastro, you would love to be there... but it's just not possible. 

4 If you can find it in you to be really brave, be honest with your opportunity.  You would love to be involved but you are struggling mentally so the best you can give is a tentative yes, is there a smaller way you can be involved? Is there a safe way for you to step back of it gets too much? You would really appreciate thier understanding, thanks.

Last I go back to 1st, you are your best barometer,  trust yourself to know your limits. You can do this - Great! You can't do this - that is ok!! Life is full of chances rarely will you find a once in a lifetime moment. If you can't this time be sure to tell them how you would love to be involved next it just isn't possible right now.

Good luck @saltandpepper I hope these thoughts help and I hope you can find a way to be kind to yourself ❤