When ever I go through a mirror in a dream, I usually end up right back in the room with a mirror in it (which is usually my bathroom). One time I found out that the room changed slightly every time I went through the mirror, and ended up in a new landscape that way, but I can never just go through it. Are there any way to get myself somewhere new. I’ve tried telling myself that I’ll go somewhere new, and asking the dream nicely to take me somewhere new, but those didn’t work. 
Try visualising the next time you try. Imagine a very vivid dream scene and maybe even close your eyes as you’re going through the mirror.
I could try the visualization thing (and maybe get back to my forest), thanks. Although, I try not to close my eyes when I’m dreaming, because after I open them I’m fully awake.