It is not advisable to sell when the market is going down, because there are many people here who bought the coins at the high value, so instead of selling those for the loss, it is good to hold until the market starts recovering.
Profit in the business is only a function of the best project up to a big ratio so of course if you have invested in a project that is not so profitable; you would face problems with getting money out of it. Better be rational before investing.