Found: fantasy(horror?) main character: Christopher. gains godly powers by building a treehouse

Christopher moves somewhere with his mom (sally or Sarah) away from an abusive boyfriend (father: suicide) he builds a treehouse with his friends (3/4 friends?) as instructed by someone Chris calls "the nice man" while building the treehouse Christopher gains powers but by using them, he injures himself. he travels into a nightmare place through the treehouse and has to stay "off the road" because some evil lady is trying to (supposedly) kill him. if anyone could help this would be great because it was a fantastic book but I can't for the life of me remember the name.

>1 Deth_Bringer: I haven't read it but it sounds like Imaginary Friend by Stephen Chbosky.

>2 Lady_Librarian84: YESSSSSSS OMG THANK YOU