As a counter-point, I have several friends who had nice companies and outstanding offers in the late 90s who could have sold but did not do so because they told themselves that they were going to make the company _huge_ and better offers were always going to come knocking (because for the previous couple years they always did.) When the bubble popped they were left holding nothing and a couple of them are still obsessed with the could'a, should'a, why-didn't-I moment that passed them by.

If you succeed once it will be easier to do again and you will not feel the same pressure to cash out because you will always know that if the current venture blows up you have a comfortable life banked away. Consider the idea that exiting now give you the freedom to exit whenever you want on future ventures by just walking away and trying something else...