selling bitcoin is not an opportunity loss. the price of bitcoin is very volatile, if the price lost it value or it goes down while your holding, it is also an opportunity loss. the point is, everything is an opportunity if you can manifest the crypto well.
I think no one knows the perfect time of buying and selling. You should learn to book profits now and then and some coins you can keep it for long term price appreciation. Because if you don't set a target then you will never sell them and always expecting more and more profit. Because we know that our mind never satisfied and always want more and more.