Believe it or not, I used to have the same exact phobia.
I thought I wasn't scared of Slenderman because I grew up in the forest and I wasn't afraid of things in the forest at night, but then I went on Tumblr and looked up the Slenderman tags, then I also looked at people playing the Slender game and I traumatized myself for a long time. It's been about a year and I'm finally able to walk around without being scared of the trees around my house at night.

I think with fears like these, the best thing to do is give it time. Your mind will eventually handle the fear a lot better with time. You may still have things that trigger the fear, but I promise you'll be able to live life without being scared of Slenderman forever.