I am disappointed since i did not cash out my bitcoin in December 2017 when it was up 20000$. It becomes really hard for me to sell when bitcoin price keeps going up

Sure, you're regretting it now, along with a lot of bitcoin holders out there. But think realistically. What are the chances that you'd guess the absolute top price of bitcoin's bull market? Even a honest and realistic expert trader would admit that a peak price almost next to impossible to predict. Stop beating yourself up for it.