Its interesting... There are some people in the world that have spiritualist belief systems. These folks like avatar here have these sorts of experiences all the time... entire nations of people. Jesus go to brazil sometime! These experiences and Avatars beliefs are extremely common there.

Avatar, your working from a fear based relationship with this entity. Has it really done anything to frighten you? Or are you just frightened of it? Things just like your experiencing happened to me constantly for years, until I started to learn they where lessons of a sort. I have heard you report it has been trying to make contact with you for a while now, nothing more then that. the feeling of it coming into you, this is what it feels like during embodyment practices or what some call possession, mediumship or channeling... personally i call it embodyment, because from a transpersonal view point we are one with all that is, all is that white light you visualize, and you cannot channel, be possessed by or be a medium for that which you already are, but you can shift awareness and BE that which you are, and embodyment is what they call this phenomena in buddhism. If your visualizing this white light then you do not know yet that thats what you are, and if you are frightened of these beings then you also do not know yet that you ARE these beings and vice versa. 

Has it occurred to you that this "entity" could very well be a tutelary being/spirit? Many are event he wrathful scary "negative ones". In many shamanic societies Shamans are contacted by tutelary or teacher spirits that teach them to communicate with the "spirit world". It seems that all this spirit wishes is to communicate. Many time when one is hearing spirits speak they hear it in their own self talk, in side the mind so to speak, but it is perceived as other or from another source. It is also often times followed by a high pitched noise, buzz or whine, which could be a change in blood pressure honestly as one relaxes into an altered state in your case pre-sleep.

its also interesting to me that your connecting this to your room mates DMT experiences. Many of the "entities" that people meet in what some call hyperspace, using "the parlance of our times" (lol) are helpful, tutilary spirits, that give guidence and assistance and teach people a great number of things, which is why i think people are so attracted to that sort of experience. Maybe it does have something to do with your room mate, or maybe it is more or less that his stories and your associations brought about a series of experiences that have been trying to teach you some things.

I do not think your mentally ill, But it is true that many pre-psychotic people or people diagnosed with psychosis NOS even when medicated have many of their hallucinations and voices occur at bed time like your describing. But because of your beliefs I do not think this is the case... they are also usually followed by delusional thinking which I have not read and can usually spot pretty quickly. And as I said many people around the world share the same beliefs and experiences you have.

Here is a simple and helpful suggestion, take it however you like. Talk to this "entity" know its a part of you just like everything else in this universe. Ask its name, what its intention is all while in active and receptive mediation when it next occurs. Be receptive and responsive. Ask then allow it to use your thoughts to respond, in other words listen to your thoughts do not think, so much. Enter into Dialog with it, see where it goes. See what happens, Its a person, just like any person.

Now if you or any one else thinks this is silly and that its all just a part of your own mind... well it is! In buddhism they have a saying, one mind many bodies. All is a manifestation of mind, but it is the mind of all that is. It has no bounderies as Ken Wilber points out, which is a rather panpsychism approach.

Secondly, spend some time developing your intuitive/receptive abilities a bit more. Perhaps find some local spiritualist or medium or psychic development groups in your area working on intuative development with out a lot of belief systems brought into the mix, and balance that out with some very serious transpersonal psychology books. Not the light fluffy stuff you find in the new age section of barnes and noble, but some books that are not focused on one belief system and that challenge and develop your rational mind and intuitive side.

some good books might be to start with,

paths beyond ego

opening to inner light or what is now reprinted by ralph metzner as the unfolding self

becoming psychic by stanley krippner

Mysterious Minds: The Neurobiology of Psychics, Mediums, and Other Extraordinary People by krippner again

and then move onto some harder stuff like

No Boundary: Eastern and Western Approaches to Personal Growth by Ken Wilber

The Simple Feeling of Being: Embracing Your True Nature by Ken Wilber

Dream time and inner space and shamans healers and medicine men by Holger kalwiet

Just some thoughts

if things do start getting really strange in your life and you start to feel very lost and confused and uncentered seek some professional help from a mental health professional.