Ive heard this many places, but what I do is let go of expectations of other people or situations all together, and its not a letting go that is in helplessness or hopelessness, it is a letting go of ego/personal/3-d hang ups in an affirmation that one is letting go and surrendering to the simple fact that god is all there is, and everything is perfect, one must just remember they are on this point in the time coaster to experience exactly what they need to experience to attain the keys to the doors of the next levels or dimensions of the ride.

Ive also heard it in a manner of:

Astudent complains to teacher that others are bothering him and stunting his spiritual growth and distracting him from meditation, teacher replies, no, you are not apprehending a perfect opportunity to grow stronger than you would in silence, only you can make yourself anything, you allow others to distract, you are the one who allows, you are the one who decides if the situation is to be a hinder or a help. Only you can decide to feed a negative reaction or not, its really that simple, ignorance is naturally hungry, feed it with nothing and it shall be full.

Letting go is a profound act that will liberate us all from anything, letting go of anger, letting go of fear, letting go of grudges, letting go of annoyance, letting go of bad and unproductive habits, letting go of selfish thoughts and manifestations. At first it may seem like an ignoring of said idea, but its exactly the opposite, it is acknowledging its divine place in the circle, it is letting go of the wheel and knowing God or Source/Love is taking it from there, you still look out the windsheild, you still know you are on the path, but you are no longer allowing things that hinder to hinder, you are then promoting such things to teachers of self, as when you let God take the wheel, you get to realize everything was already perfect, and your ego is all that was suffering, you do not suffer, ever, only the illusory constructs the ego creates do.
Even in physical pain, there is a letting go or rising above that some may not experience until severe pain at which point often one can feel euphoric or nothing at all. There are even folks who use pain to reach such states of awareness, granted things like body suspension may seem sort of extreme and self destrucitve, for the student of the mind they are effective. The point here is, that you dont have to go to the extreme of putting your self in pain(suffering) just to overcome it, or just to understand it, let go and take a carefull look at what it actually is.

Now about manifestation ("the" secret) and letting go, there is an important key here as well. When we so wish to manifest a different reality, one that benefits not just us but those around us, we must hold it firm in our mind what it is, and then let go. This means dont try to trim it up too much or add too much to it after the initialmanifestation. The more clear and precise the thought, the more clear and precise the manifesting is. Also the let go here is like the letting go of a seed into the ground, you dont dig it back up to see if its growing right or if it landed correctly, you just water it each day with the same intention you had in planting it, holding that firm thought in mind and visualizing how it will change the lives of beings due to its fruit it WILL provide. This luckily is a "green" thumb that everyone has, we already do it all day by acknowledging emotional states of mind. For instance, if you wake up on the wrong side of the bed, just walk to the other side of the bed and decide to wake up there. Let go of the destructive attemps of negativity, and wash them away with confidence that positivity is your natural constructive preset.

-Psychedeliack