It is gambling even if you say that you know what you are doing. Your above example isn’t a good one for me:
For example, you buy a few bitcoins at a December high and expect the price to continue rising, which is kind of obvious. Instead, the price starts crashing down and you find yourself in a situation that you didn't envisage or consider beforehand. So your best option would be to bring things back where they were as fast as possible even if it means some loss.
As a long-term holder, I just HODL. That saves me a lot of headaches and a lot of trading commissions. You just put an example of what newbies do: to buy after the last ath out of FOMO and to sell after the dip out of panic. This is the opposite of what I usually recommend.
Actually, I have no problem with this approach. It is your choice and I'm okay with it. In fact, I also follow something similar with coins which I think are worth holding. But you seem to be missing the whole thing I'm trying to demonstrate here. It is not what you do specifically, selling or holding, or whatever, it is how you do it, whether you are doing it because you have no other option left and out of despair, or it was your choice planned beforehand. This is where I'm trying to draw distinction here, not between certain actions like closing or sticking to a losing position.