Assumptions are interesting things. When we assume something about someone we see them through the lens of that assumption or belief. And as the old saying goes an assumption or, more properly, to assume is an ass between you and me.
But when we are creating intentions we actually want to assume that they will come to be. This in an interesting part of the creation process that we need to take on board if our creations are to come to pass. In doing so we also need to feel and experience so strongly the absolute rightness of our intention that with the power of the Divine within us we cannot help but create the situation we want.
In the process we are using our Divine being within to create a new state of understanding of ourselves. It is like we have to step into the person we will become in order for that intention or desire can come into existence. We have to walk, talk and yes, assume that it cannot but help to happen because we have commanded it into existence from the magical part of ourselves that are part of the Divine.
Saying a Command once, desiring the thing intensely and then falling back into our old way of thinking, feeling and being is a sure fire way of it not happening. You have to believe, or assume, the thing is coming into existence even at this moment and then live your life as the person who already has it. Then you must allow your faith (the belief in the thing unseen in the physical world) to lead the way.
If you have used affirmations in the past and they have not worked, it is probably because you are trying to do them with your mind (which is only one part of the equation) and have forgotten that to reach the powerful subconscious part of you that does the work works through feeling. In this respect your intention, coupled with the feeling that the thing exists already, right now, is the magic that allows it to happen.
Then you just have to allow the Universe to do its magic and remember that the Law of Gestation is often involved in the bigger things in life. So you have to wait for the Universe to organise its happening. Sometimes it takes almost no time at all, and sometimes it can take much longer. Continue to live your life, knowing that it will happen in due course, not in your time, but in that of the Divine. Then go off and live your life finding as much joy and fun in it as you can, because those two emotions are the grease the Divine works with.