This may seem very obvious, but some people do not seem to clearly understand what selling means. For example, I know a few people for a mid-size village that, during a housing boom, sold quite a bit of land to developers and made a development plan that was pretty much crazy (high rise on small streets, minimal walkways, no space for public services, ...). They were very happy about getting the money. Now, all I hear is complaints about how many disgusting people that are not born there came to live to "their village".
When you sell, is no longer yours. When you sell your bitcoin for more than you paid, you may be happy about it, but unless you have an investment to make or would like to buy something else, all you have left is... fiat. And fiat finds its way out of pockets very quickly.
It is obvious already that when you sell your bitcoin, it will no longer yours. You've decided to sell it, so, you don't need to regret something after doing that thing. We should be ready for anything after we sold our bitcoins. Selling isn't a bad things, in fact, we can gain profit through this. In fact, that's the point of selling, you're giving it away to someone through a trade of goods and services or something else, that's how selling works. That's the reality, sometimes after selling bitcoins, you will regret it when you see prices of bitcoins are increasing but it isn't bad yet since you sell it for your own good and with a good reason.