I heard it's possible to leave a portal open for malignant spirits and evil to enter this dimension. As such, is it also possible to leave a door open to once's own mind? If possible, how and/or why would this happen?
I think I did it, but I don't know how it happened in the first place. Ever since then, it felt as if I'm not really sound of mind, as if there are rare moments of peace. Something must've happened, because I was never like this. I was a lot stronger.
Perhaps it's magick related and perhaps not. I've noticed when alone I sometimes act..strange yet I chalk it up to boredom. We don't know your life so you may be going through issues that are affecting your perception of yourself and the world. In the end if you did cause this unlikely phenomena you can stop it. An open door can be closed and bolted.
I've never heard of a door of the mind being left open to invite possession. Perhaps it would invite mental attacks but not possession, if I understand your question correctly :) To prevent such mental attacks, I would add mental protections to any physical ones you set up.
Possession happens when a stronger consiousness decides to control the body in that a non-stronger consiousness exists. It can happen anytime and there's no "door" for that.
He may meant possession as a way of talking with spirits; through possession. This is possible, it is called as chanelling or psychic mediumship. In channeling essentially the entity comes inside you and gives you messages in certain ways. It is said a paper is used for the entity to show messages, but I don't know if the entity can talk from the mouth as well.
In psychic mediumship you are in a trance state, the entity enters you and talk through your mouth while you focus on an object. I have a friend whose her grandma does this. She fofuses on a candle flame. I don't know if something different happens here.
I don't know also if there is another way to get "consiously possesssed".