This is an interesting concept I came across six years when I was exploring estoeric knowledge on the internet. Back then I hadn't really understood what it was really about but now, after observing a few life experiences, I have started to get it.
The Law of Assumption states that whatever you assume as true becomes your reality.
The law was introduced by the author and spiritual teacher Neville Goddard (1905-1972).
This law is such that it only works if you have 100% conviction that what you want to happen has already happened and now you are living from the end. (how you'd live and behave if it were to have already occured.)
It may sound strange to the logical mind that such a thing could be possible. But the logical side is just one side of the coin. The energetic aspect is another powerful side of the same coin that remains hidden.
Ultimately, everything is goverened by energy and when you apply the law, the energy within you shifts and the desperate energy loses it's control over you.
This ends up leaving you with an open mind to possibilities and you won't impose limitations on yourself as to what's possible and what's not.
Applying this has done wonders in my personal life multiple times. I won't get into the details, but it's definitely had an impact on my life.
Remember one thing though, that you need to have 100% conviction that what you want is already your new reality.
If you have even an iota of doubt, it won't work.
Done.