Clearly making big money, why does the account still end up losing in the end?

After trading for a while, I increasingly feel that there is one type of account that is the most unfortunate.
It's not that they've never made money.
It's not that they never buy.
It's just—despite catching big opportunities and earning quite a bit, in the final tally, they still don't keep the gains.
There are actually quite a few people like this.
They have their own stock buying logic.
And they have indeed made profits relying on this system.
Sometimes they make a big profit, to the point where it seems like: this method has become foolproof.
But the problem lies right here.
Being able to make a profit once and having an account that ultimately makes money are actually two different things.
Many stock trading theories address questions like:
Why do I buy it?
Where might it go up?
Is this opportunity worth taking?
But whether the account can actually keep the money at the end depends on something else.
It depends on the entire follow-up system:
How much to buy
What to do if you’re wrong
How to take profits after earning
How to slow down after a loss
When to stop in different market conditions
Which opportunities shouldn’t be pursued anymore
Many people understand the first part quite well,
But the real problem is often in the latter part.
For example, after making a big profit, they start to feel confident.
They increase their position size, trade more often, and become less tolerant of mistakes.
Trades they would have hesitated on before now seem “okay to do.”
Places where they knew they should take profits before, now they keep thinking “just a little more.”
The most common result in the end is:
Money earned earlier through skill, is gradually given back with “ability.”
So now I pay more attention not just to whether a system can help me make money,
but more to whether it can truly help me keep the money I’ve earned in the account.
Because as I trade deeper into the process, I increasingly believe one thing:
Finding opportunities isn’t the hardest part.
The hardest part is making sure you don’t give back what you’ve earned.
Many people aren’t incapable of making money.
They just can’t keep the money.
And “not being able to keep it” isn’t usually because they’re not smart,
or because they lack theories,
but because they only have an attack logic and no account system.
What I’m becoming more cautious about now isn’t “whether this trade can make money.”
It’s more about:
If I make money, do I have the ability to lock it in?
If I’m wrong, do I have the ability to exit?
These two things truly determine what the final account will look like. $BTC