Somehow, this movie managed to be intense while simultaneously a total letdown. It's an allegory about the personal struggles of someone who wishes they could transition to their desired gender, but instead crumbles under the pressures of society. Let it be clear, the subject matter is not the real problem with the film: if you, "were expecting a horror film that wasn't preachy" then that's a YOU problem; art can be appreciated for what it is, even if it wasn't what you expected going into it.
No, there are actual problems with this film. It's horribly paced, brushing over important details with quick strokes while slathering on other details that really don't matter. The relationship with the father deserved more attention; the expectations of the parents and society deserved more attention; the mundanity of living life under the wrong gender role deserved more attention throughout instead of being crammed into the epilogue. The reuniting of the two friends as adults felt abrupt and came to an even more abrupt end.
Ultimately, it was a heavy topic that was executed heavy-handedly. As someone who isn't trans, it takes a little bit better writing and directing to get me to sympathize for the main character. I'm sure a lot of viewers would agree.