I'm perfectly OK being underwater for a while

This is the right mindset for long term Hodling and guaranteed gains in a still undervalued crypto market.
You pretty much can't lose, yet. Unless Bitcoin crashes for good. Which I don't think will happen in the near future.

How do you deal with the anxiety of wanting to trade or sell partially at peaks and take some profit?, if you even have that anxiety. Well, I guess trading with the borrowed funds probably takes care of those needs.