I am an entrepreneur based in San Francisco who has spent 3 years building a healthcare-related company. I have dropped $150K into the product. I have a few customers but no traction. I feel like giving up, but $150K is a hell of a lot of money to have put in.
Best advice I've ever received:
"Never throw in the towel because of exhaustion. Throw it in because the model doesn't work."
Until you prove the model doesn't work, keep going.
I nearly gave up, without proving my model did not work, weeks before I raised $100,000 in funding. I'm glad I listened to this advice and kept pushing, even through the tough and discouraging times.
It's easier said than done, I know. I've been there. You know how to reach me if you need me.