I read many regrets about not buying BTC when the price was still cheap.
And I am sure that everyone also did the same, regret it, not buying BTC at a lower price or regretting selling BTC at the low price.
For me, I didn't regret it because I really couldn't do/buy it. Moreover, I also sold my signature reward (BTC) to cash because really need it for my family necessities and life, so only very very few amount that until now still holding. 

The most thing I regret is always delaying to learn future trading. I start learning future trading and doing it, but always stop at the middle because of some reasons and always canceling my willingness to learn it. 
And until now I still don't start doing future trading again because I really don't have enough skill for it, need more learns. because I realized that the following signal from others is not always good

Sorry to hear that, it can be a very precious lesson for you personally and for us, gambling is not always good for somebody, mostly we only will lose our funds if we really cannot afford it and have no enough skills to do it.