Morton's fork: Choosing between unpalatable alternatives.
Sherlock knew that there was no right answer here. The could be no right answer.
Either he jumped off that building, convinced John he was dead, and convinced everyone he was a fake, or everyone he cared about would die.
There was no easy answer.
Delay is the deadliest form of denial.
But he couldn't just stand there, frozen. That wouldn't do.
But really? His own suicide, even if faked, would cause immeasurable pain to John mostly, Mrs Hudson, Lestrade, and possibly even Mycroft.
But if he didn't, Lestrade, Mrs Hudson, and John would all die. (John. His only friend. The only one who had every really understood, who had taken the time to get to know him, who actually seemed to like him. He would die. And there was no way Sherlock was going to let that happen.)
And when he looked at his choices like that, they weren't choices; not really.
And in the end he chose the one that was the only option he could have ever chosen.
So he called John, cried to him, tried to explain in terms that meant something different, and watched him as he realized what was going on.
And he jumped.