I have been in a similar situation. I was a co-founder at a company, I was not joining with technical skill. I had more marketing and business development background. I had some knowledge but was just starting out.

The opportunity was fantastic and I learned a lot (things todo and not todo). However, as you stated I felt the exact same things you did:

A. Learning Was beginning to stagnate - I was no longer able to continually learn and develop.

B. Company was not growing and my confidence in the rest of the team plummeted. We were focused on the wrong things and promising things to customers and partners that we knew we could not deliver on.

These two things alone where enough for me to know that my time was done. There is a post on Medium, "Rate-of-learning: the most valuable startup compensation" (link at the end) this is why I had to move on. I knew that for me to continue progressing and learning the best option for me was to leave.

In regards to equity and raising capital. I learned two things (1) people can become extremely greedy when equity and raising capital come about and (2) greed can easily lead to ethics and values being compromised. I don't know your exact situation but I would keep these things in mind.

Hopefully this helps. Best of luck!

Link: https://medium.com/@KyleTibbitts/rate-of-learning-the-most-v...

PS: This is another post that may help you by Ryan Hoover. Great post! https://medium.com/@rrhoover/knowing-when-its-time-to-move-o...