As you’ve discovered, being friends and being business partners are two different things. You are correct that the two of you are in a destructive pattern. But it is a mutual dance. And it takes two to make a cycle. That’s why it is called a “cycle”.
I don’t think the two of you can solve this on your own. If you could, you would have done so already. You need new eyes on the problem, preferably experienced “eyes”, who can help the two of you get out of a destructive pattern and into a productive pattern of relating.
I therefore suggest you get a consultant to help you. No, this doesn’t have to cost money. Check out the website for SCORE: https://www.score.org/. SCORE provides free business advice through a network of volunteer business experts. You can get help by phone, online, or by arranging to meet with a mentor.
The stakes are high. Most start-ups fail. When business partners don’t get along, the business inevitably suffers and often closes. I hope the two of you can get past whatever ego issues might get in the way and reach out for the help you need.
I wish you well.