I can not find the original quote in Chinese. It may be so difficult because the original text is probably very far from "When I let go of what I am...". Does anyone know what the original Chinese source is saying?
It is hard to find because it does not exist. This quote, along with "When I let go of what I have, I receive what I need", are made up by John Heider in his 1985 Tao of Leadership. Here is from Taoistic:
"This fake Lao Tzu quote is weird not only in the eyes of Lao Tzu, but in those of just about every ancient thinker. They would all agree that I have to be what I am – that's the path worth taking. What I might be is just an illusion, making the mind fool itself. It makes sense in our modern zeitgeist, though, where everyone from a young age wants to be famous and spectacular. Like in the credo popular in the American culture: “You can be anything you want to be.” Good luck with that.
[...] Both lines are from a wildly tendentious version of the Tao Te Ching by John Heider: The Tao of Leadership from 1985. It seems to have no intention of being true to the original. The fake quote is very loosely based on lines of chapter 22 of Tao Te Ching. So loosely that it is difficult to ascertain exactly what lines he interprets that way. Heider seems to give those two sentences as a kind of digest of the whole chapter, and then he moves on to explain what he thinks it means. He calls them feminine/yin paradoxes."
Wikisource gives the Mandarin original alongside a translation. If there are lines that summarize chapter 22 it would be these:
"Hence, the master maintains integrity,
And sets an example for the people.
...The master contests not,
Therefore is uncontested.
... With genuine integrity, one rediscovers self."
Here is Taoistic's translation of chapter 22. The same lines are:
"Therefore, the sage embraces the one,
And is an example to the world.
...Because he opposes no one,
No one in the world can oppose him.
...Indeed, he shall remain whole."