Everybody needs a break, no one is superman and threfore cannot go on forever without a break.  It is important to take a break, switch off and recharge the batteries.

A break might be a short nap or even a proper holiday somewhere in the sun 

I have just done exactly that -  the holiday in the sun that is  and I have to say I am much better for it. 

You need fortitude.  Things will probably not go your way initially and you will need be tenacious and persistent.

 
A very interesting Wall Street Journal article really resonated and articulated this point by illustrating initial failures of public figures such as  J.K Rowling, Julie Andrews and Michael Jordan.  I have always understood the need for tough skin but this trait was better described as self-efficacy and the key is to manage failure.

Working on a start up there are many failures and challenging moments and managing those must surely help to be more successful.