How do you break-up with (50/50) co-founder after 2 years?

I have been with my co-founder(CEO) for more than 2 years now and I just completely lost faith in him and I don't believe is he capable of being a CEO, I don't have any confidence left in him to do sales and I don't like the way things have been working out. We have been stagnating for the last year. I tried many things to change the situation and I just don't want to keep going with him.

Now we have some investors, revenue, interns partnership agreement,etc.

Recently we let go of one of the late co-founders and we signed a new partnership agreement and I didn't check but it says that vesting period starts from like 6 months ago. Instead of 2.5 years. So technically If I leave now I would loose everything. Also Why do i have to leave when I'm not happy with him, right?

Never been in this situation and I'm looking for advise...

Thank you