Either find a smaller role in the company, and still be supportive. Do that if you can. If you can’t, if you really don’t want to stay. Then rip the band-aid off.
And just leave.
It’s OK. It’s for the best.
The company may immediately lose its funding. It’s terrible. But it’s OK. If the CEO is really great, she’ll find someone else. More committed that you. Better than you. Then somehow, someway, find more funding. In the end, the company will be better off, no matter how much the short term pain.
Stay in a reduced role if you can. If you can’t. Then.