Oh i know how you are feeling and I also know just how lonely it can feel too. My teenage years were also very , lets say edgy and panicky.
I also understand about how difficult it is for you to tell your mum, my mother was difficult to talk to and would often dismiss things i said as being silly.
You're going through a lot of change both physically and emotionally during this time of your life and I am here to tell you that you are not alone and that you would be surprised just how many people of your own age probably feel similar.
My advice is this, find someone you can trust and talk to. A best friend, a favorite teacher, another close family member. let them know how you are feeling. Just letting it all out can be a great stress buster. having support and someone you can turn too is of enormous benefit. After all therapy is not that dissimilar, they won't have any magic words for you but they will listen and so can friends etc.
i know it will be hard at first, but just tell one person, you'll be surprised just how supportive people can be.
Also try to find things that you can immerse yourself in, something you love and enjoy. Do it regularly and if it involves a group of other people the better. perhaps you like sport, join a team or fitness club. I know this may all sound rather obvious but believe me I have been there and i know what works.
Good luck and don't worry there is light at the end of the tunnel.