I think every knowledge worker should have the investor mindset.
You are not investing money, you are investing your brain and time.
The output you might expect is exactly the same as the investor expects : money, making the world a better place, and making the best use possible of your input. (money/brain/time)
Accepting a project is not about convincing the customer or partner.
It is about making conscious investment choice.
Is it worthwhile ? useful ? profitable ? when are you going to have your first customer ? What is your team ?
These are the same questions as the investor. Your goal are the same, even though your resources have a different nature.
However be even more careful about the team : you're gonna spend long time every day with them.