You have seen Slenderman 13 times? Is there any way that you can substantiate the sightings? After all, this is the internet - anybody can say anything, making everything ANYONE says suspect.
You can say its not imagination - however until you can back it up with some sort of proof - your words are just words. I mean no offense in saying that. You have got to realize that saying you saw a thing proves absolutely nothing.
If what you are seeing is causing you distress and making you feel as though you need help, I'm quite sorry to hear that. I'm sure that it seems very real to you.
So if I may inquire - before you had your 13 encounters with this Slenderman character - you had never read about, heard about, or seen anything at all related to the paranormal that would have shaped some sort of belief in your mind? You didn't believe in spirits, ghosts, demons, monsters, ghouls, djinns....etc?