Learning from mistake.
Our idea was to allow mobile data sync and we had prototype ready. We submitted our application during one of the YC round ( Last year ) and got rejected. We don't have other sources to get funding so, we stopped working on that idea. We stopped because our idea needed funding to go to next stage ( For testing multiple phones etc. )
Recently ZYB got sold for @49M.
Well! We also got rejected for other idea ( healthcare related) in last round and we are still working on it and planning to apply again for next round :).
Don't give up!. Print YC adv. and keep it near your desk to remind you your end goal.