It all comes down to these kinds of situation in which our trading skills are put to the test, anyone can make an easy money in the bull market but bear markets are where our skills are developed and our ability to make future profits is carved.
In a bear market, all coins are at its cheapest so it is a good chance to take some money out in the bank and invest it in a good coin..
It is truly hard to see that you didn't sell the thing in time and now you keep losing. I think this is when people often make a mistake and sell the coins even though they lose from it. They sell, because it feels like the drop will keep moving the investment to zero.
What you say about now being the great time to invest in coins is true, but I believe that trading should be distinguished from it. While now is indeed time for investing and holding, trading might be rather hard right now. Trader should not care about the value of the project, should not believe in some and keep holding when the price is going down. He/she should simply buy and sell regularly and on time. Nowadays the volatility is pretty good, but the coins often just keep going down, so trading would be a truly hard activity in such conditions.