Hi Charlotte
What stands out most to me is your age. Many times "adults" will negate very real feelings of younger people, saying it will pass. Sometimes it does and we call these feeling growing pains.
I think often the adult might feel inadequate to properly address the teen's emotions. They might recall instances they experienced which were passing discomforts and the easy thing to do is brush off your feelings. Many adults don't deal with thier own emotions well and seeking help from someone in this predicament is like trying to get milk from a bull cow.
Septembers advice is sound. See if someone at school can help. There are different causes for depression. For some, it's a medical condition which they can't control but medications can make life liveable, even great. For others, like me, depression is a reaction to oppressive conditions. It's important to know the difference. Knowing that we can change our perception of things in our environment or change our situation is key to treating reaction type depression. In either case, you'd need a trained professional to help you understand these confusing and similiar symptoms.
Best to you, keep us posted, ok?
"Our greatest glory is not in never falling, but rising every time we fall." Confucius