if in 2019 will begin a fast rise in the price of bitcoin, then most likely I will sell bitcoin when it reaches 25-30 thousand and then wait for 10 thousand to buy it again
Sometimes, you might not be sure that the price you are waiting to be will be the lowest. That was what happened to some investors, who after bitcoin plummeted, after the ATH, yet it kept on plummeting until the present price.
I believe investing now is still the best.