The only time you hold (hard) onto a dream is when you believe you can't have it. If you knew you could have the cupcake on the counter when ever you want, you don't really go on daydreaming about it for days and days and days. It's only those desires you've made up in your mind as BIG or hard to get or whatever other resistant story that you keep holding on to. So YES let go of the dream once you have dreamt it up - you don't have to keep dreaming it to make it true. You have dreamt it because it is already true in some dimension of this vast pool of vibrational patterns. You have dreamt it because it already exists. So you don't have to keep on dreaming it, you don't have to keep on keeping yourself in the asking step.
We make castles out of buttons. Ever notice how just a passing desire manifests much more quickly than a desire you really really really want? The Universe hears you INSTANTLY - you don't have to hold on to your asking to receive. Recognize you have a desire, feel it, see it, bask in it, and then let it all go - let everything about it go, from the wanting to have it to the wanting to resist it and everything in between. Exhale every bit of it out of yourself. Let the energy you have recognized as a desire and have activated leave your personal energy field, let it ripple out of you and into the Field. And then go on about your days. Trust this flawless design. When you ask ONCE, it is already GIVEN. Do you trust that? Or does it feel like you have to ask the universe over and over again, that you have to daydream and virtual reality about your desires X number of times before the Universe says "your wish is my command?" Check in for how you've set it up for yourself. The Universe will play with you within the parameters and rules you've set up.
The Secret is in the letting go.