I always forget what I am doing

There actually is a reason for this is called the event horizon model. Basically your brain renders things in episodes or segments not continuously. The act of walking through a door or even think about it seems to insert a mental divider in our memories, like pressing pause on a video. And in that short amount of time you can forget. And your brain also thinks that you are finished with whatever you were doing in the other room which included thinking about what you were about to do and starts already moving on as you go thru the door way. I find starting focused on what you need to do as you go through the doorway always works for me.