Too valuable to destroy, too dangerous to give.
The other answer is painfully obvious.
You must be custodian of this discovery and keep it secret until it is safe to give, until mankind has expanded to the stars and has new worlds to populate, or until, even without that, he has reached a state of civilization where he can avoid overpopulation by rationing births to the number of accidental—or voluntary—deaths.
If neither of those things has happened in another fifty years (and are they likely so soon?), then you, at seventy-five, will be writing another letter like this one. You will be undergoing another experience similar to the one you’re going through now. And making the same decision, of course.
Why not? You’ll be the same person again.
Time and again, to preserve this secret until Man is ready for it.
How often will you again sit at a desk like this one, thinking the thoughts you are thinking now, feeling the grief you now feel?
There is a click at the door and you know that the time lock has opened, that you are now free to leave this room, free to start a new life for yourself in place of the one you have already lived and lost.
But you are in no hurry now to walk directly through that door.
You sit there, staring straight ahead of you blindly, seeing in your mind’s eye the vista of a set of facing mirrors, like those in an old-fashioned barber shop, reflecting the same thing over and over again, diminishing into far distance.