The “play pretend”
is the world she inhabits while high. By characterizing her high(s) in a child-like manner (“play[ing] pretend”, the “playgrounds” mentioned in verse 2, and endless “fun” are stereotypical juvenile desires), the narrator subliminally conveys a realization of the immaturity of her actions.
When she says Can't go home again
shes saying she can't go home, although she is having fun, she still misses the guy she is talking about in the song and someone needs to numb her pain.