Building a successful startup is infinitely harder than having a successful career as an employee.
Because building a successful company = building many successful careers. The point that many companies still do not get!!!
You absolutely have to invest in your people in a best possible way if you are interested at all in building a GREAT company.
Conducting a successful career is also very hard. Let alone a successful marriage! Or getting fit, loosing weight.
You want to build a company? Just do it. It's not harder than other things in life.
Edit: some words.