The Slender Man, is only a fictional character that originated as an Internet meme created by Something Awful forums which is not really tied to any particular story,
only appears in many disparate works of fiction, composed online by Eric Knudsen/Victor Surge in 2009 only, it is not even an ancient myth.

Since this fear from him and the forest came out of reading,
then, I'd only suggest to write what it is you found fearsome about the character. In short, just write a story about this character.
And whenever you find yourself scared, tell yourself aloud that "Slenderman is only a fictional character".
The aim of the story for you to write about
is actually to turn him into somebody who is totally boring,
(Why? To simply psych yourself into not being bothered by him,
much like the story of Harry Potter's confrontation of the boggart, see http://www.youtube.com/watch?v=6hxF65X029c
ok, this example is a bit exaggerated )
or if you have the inclination, turn the Slenderman into a hero. Just don't forget to include the forest when you do - makes a good ending, too.

One more important thing to do is, think of something brief which you find good and happy, do so five times while saying "forest",
then, whenever you hear the word forest think about this happy place after that.

Now, to strength.
Some people gets a surprise where they find their strengths, but let's just keep this simple.
Presumably, you know what you love to do, & where you do relatively well.
That is the best place to start,
take that passion and draw strength from it.
I suggest a brief meditation to declutter and clear your mind,
Relax and have some peace. Let this peace envelope you. & let this peace be the ground of your strength.
If you believe in guardian Angels, you can ask yours to watch you and help you through it.

It actually is just simple to "have faith in yourself".
It is the same as knowing what your passions are, what you can do.
In the face of anything fearsome,
know and remember that your soul and life is yours, not the devil's. Remember the story of Job of the Old Testament?