Hi Jaap,
Good for you, the fact that you are aware of the risks of your emotions getting in the way of business decisions is a great start. I would say that the very best tools you can have are a firm budget and milestones. You should decide early on how much of your own money you are willing to put at risk or, how much time, or both. Write these things down, share them with a partner and make sure you hold yourself accountable. Too often entrepreneurs move the goal posts when they don’t make their milestones. Avoid that. If you reach a point in time and you have not accomplished your goals and you have spent all the money you had budgeted then it is time to stop and rethink the project. Good luck and I hope that helps.
Best,
Joel