Hello,
I completely can relate to you with this, I have been feeling this way since I was 14 (I'm now 19) and I understand how hard it is to open up to your parents and worrying what they might think or about disappointing them.
My parents didn't find out until they got a call that I was in hospital after attempting suicide when I was 15 - they were so worried but also so supportive it made me realise that I'd let it go on too long and I should have opened up, after all your Mum loves you and will always be there for you. You don't have to worry about letting her down or that she will think you're being over dramatic, take it from my experience.
Even now my parents are so supportive, even though I've grown up and moved out and working they are still always there for me and checking how I'm coping.
For now, here is a brilliant place to vent and find support and advice. Everyone is so kind and no matter how silly I think my problems may be, there is always a whole load of supportive wonderful people here to help you through it. What did your Mum say when you asked about seeing a therapist? Did she say she would consider it? It always helps to have someone you can open up to who knows just what to say and how to help. Big hugs xx