The title may have had the word "forest" in it but I DEFINITELY remember the cover having a girl walking into a forest with her back turned.
The story went something like a girl being at a boarding school I think because I remember her living with a roommate in the forest. There are these two brothers and one of them has curly hair. And the first brother is who she likes. Eventually he reveals he's a vampire or something like that and so she allows him to draw blood from her. And I explicitly remember that the blood was drawn from above her armpit.
In the end she gets together with the curly haired brother who is also a bloodsucker and he receives blood from her roommate.