Even if your cofounder has absolutely the best intentions, not everyone is cut out for it. It's very easy to be "Saying the right things" and not even aware they they aren't being honest.
In fact, I'd argue that %50 of the "co-founders" of YC startups are not actually ready to be founders, and I suspect that the number who are still with the startup 3 years after are very small. (and that a large number of the YC companies that don't exist after 3 years don't exist because of a bad choice in co-founder.)
- deals that made a loss - poor relationship management - investing way too much in blaming the team around her
This ended up with a company seriously in the red, heading to liquidation and a completely fucked reputation.
Iteration two: flying solo, small team and 18 months in, we are in profit and I've fixed most of the relationships that needed some help.
I have a department head who I can trust and points out my blindspots but never wanted to be a founder, which is fine.
Be very careful your cofounder isn't just saying the right things in the hope of a payday.