Im just a small dolphin, OP.
I only sold btc to pay my file cloud service. Regretted it, should have used fiat instead.
I also sold some to buy altcoins, but most of those were eventually sold to buy bitcoin back. So no harm here.
Never shorted it in a fiat exchange, though. Been thinking about it, but its too risky.
Fortunately for me, I started fiat shorting recently. It is a whole lot risky, but with some huge amount, one can be making some pretty good amount of cash daily even though it can be crazily stressful. You do not have to regret anything, what you should do now is to make sure you do everything not to regret the future. We cannot always win every time and life itself has left room for us to make some mistakes and learn from it.
I have been holding my bitcoin for a long time now and despite the fact that some people keep singing rubbish into their own ears not mine concerning bitcoin being a bubble, I just feel they are extremely butthurt, they could not buy when every other person was busy buying and they wish we could drop it back at that value for them. Their wish!