Assuming you have sufficient BTC's... why can't you just withdraw your initial investment and enjoy that? The rest you can leave "invested" in BTC to sell at a later date when it is at a price that satisfies your "greed"

You do raise a good question though and I think this is a dilemma for a lot of different investments (not just BTC). At what point do you cash out? Do I take my profits today and then regret that decision tomorrow if BTC doubles in price tomorrow? Do I take my profits today and be thankful when BTC halves in price tomorrow?

Personally, I think the best thing you can do, is set a target that you are happy with... and when that target is reached, cash out. If you wish to reinvest in BTC, then forget your previous investments and start from a new "zero" point. Otherwise you will always be second guessing yourself and wondering "what if?"