The key is staying positive on the long term outlook. My brother sold half his bitcoin a month or two ago because he said he didn't wanna stress over it. I told him that was very stupid. He only had a bit over 1 bitcoin anyways, so even if he lost all the money its not like he'll miss it in the long run, but if he had stayed positive he might be sitting on well over $100k in a few years. Just gotta keep a positive long term outlook. Sure it sucks getting trades stuck for a few months or watching your portfolio go down for a few months. But if you wait long enough 99% change you'll be making lots of money again in the relatively near future.
Staying positive may be good if you wanted to focus on long-term investments - in fact it looks like the only option to stay just sane in the long run - but this is not what I wanted to say in OP. It is not about talking yourself into doing nothing for a couple of years in a less stressful way. Quite the contrary, my whole point was about carrying out a well-thought-out plan how to get out of losses if you were unlucky to buy at December highs.
I think your mentality going into a highly volatile investment like Bitcoin has to be "hey there may be times when I'm way down on my investment but I believe in it long term and I only put in what I can afford to keep in there for a long time so I don't care about a market crash, I can just wait".
If you don't have that mentality and strategy then you shouldn't be investing in Bitcoin. That's how you stay positive no matter what the price.
I don't think it works in practice. Indeed, if you invested a relatively small amount of money (relative to your income, of course), then you kinda have this mentality right away. But even in that case it is not guaranteed as the other poster justly noted. But if you invested in Bitcoin all your lifetime savings, actually having this mentality would be bordering on insanity. Basically, what I talking about here is a technique for setting yourself up when you can act rationally. It is definitely not about making you into a Bitcoin holding zombie.