I'd say chill out, the project is probably not that big of a deal in the scheme of your life. I've been in a situation like that and most of the problem is your own thoughts reinforcing themselves. In reality you can pause work at any time. You can drop out of the project entirely. All it takes is saying "sorry, I'm not going to work on this any more."

So talk with your marketing partner and tell them you're working too hard and need others to help you more, and that you're going to take a week off from stress. Then try to think about other things. Go see people you haven't seen for a while. Come back after a week and see how your partner can address the work imbalance and see if you want to keep going with them. Seriously, just do it.