This forest you speak of is the forest of your memories, each tree represents an event in your life. Normally this mist that you see in the forest of memory is a white mist so that you don't overwhelm yourself in all the memories of your life, but your darkened mist means that something really bad had occurred in your past, so bad your mind has made this easy to walk through mist a place where you cant find anything if you travel too far in. I've used hypnosis techniques that allow me to enter my own wonderland with crystal clarity and have a guide of sorts that tell me what locations are there and what they represent.
This thing in your mind resides in most minds, its called your shadow because of what it represents. It's made of the things in your 'original' personality that you have attempted to remove from yourself and replaced with a better, but unnatural trait. It is also made from things you fear and despise, causing it to make you feel extreme anxiety and dread. It is that thing that you feel staring at you in the middle of the night, filling you with fright. It is there as a byproduct of the mind and is there trying to 'protect' you from other sentient things and past memories that might try to invade the mind, but instead it attacks you and your tulpa because believes it to be of foreign entity. Its sort of like a auto-immune response, but with the psyche attacking itself instead. The shadow can attack on its own accord if it feels its presence is in jeopardy or is being 'replace' by a tulpa. There are ways to diminish the power of the shadow (though still unable to 'kill' it) and have it never cause what it does to you ever happen again.
You need to conquer your fears, accept some of the parts of your 'original' personality you were born with, and if it does try to show its face again, do not fear it for you are the one who made it (albeit unintentionally). You must confront it with or without a tulpa present to help you. The stronger it is, the better chance it can overcome your desire to create a tulpa and even if a tulpa is created, destroy the tulpa. So in response to question two, yes, your tulpa can help you fight this threat, just make sure that you do not let him/her/it touch the shadow, for its power will 'sicken' and corrupt your tulpa in the most vilest of ways. Your tulpa can recuperate if you try to save it, but it will either forget what had happened between the time of contact and the end of the recovery period (it need to be unconscious for this recovery to work, and I say its for the best for your tulpa to be in this scenario) or it will change in unpredictable ways. Your bad memory is what is corroding and darkening your forest of memories and has also is feeding the shadow most of its power. I would suggest entering this forest (though you need to change it back to the 3d format like you had before) and prepare to face this dark mist along with the shadow, but I am unsure because of how bad the memories are and how strong the shadow. It would be easy to back out now, but if you defeat this darkness, most if not all your problems will be gone for good.
I am writing this with a state of mind that is kind of spacy, so if some of these things seem confusing, just ask something about it and I'll get too it.