I've been feeling nothing for a long time and I'm tired of pretending that everything is fine. Need advice or just support

I'm 17 years old, and I study at a college. For several months now, maybe even longer, I have been living with a feeling of complete inner emptiness. I feel neither joy, nor sadness, nor anger - just a flat “nothing”. I can't cry, even when I know I should be hurting, and that makes it even worse. It feels like there's a huge black hole in my chest and silence in my head. It's more exhausting than any strong emotion.
On the outside, my life looks normal, but inside everything is very hard. My parents moved to another city and live separately. We barely talk, and I feel like they don't care about what happens to me. My older sister, whom I live with, doesn't really get into my problems either. And I also have a close friend with whom I have a very heavy, emotionally draining relationship. She has a lot of her own problems, including self-harm. I am constantly trying to support her - sometimes it feels like that's all I do. I've become some kind of a rescuer for her. This has been going on for a very long time, and I feel completely burned out.
The hardest part is that everyone around me thinks I am strong and that I'm fine. I am good at putting on a face, smiling, joking, supporting others. But inside, I am empty. And it's very lonely - when your pain is so invisible that nobody asks how you are doing. I keep everything inside. Because of this state, I started having thoughts about hurting myself - not for relief, but just to feel something, to prove to myself that I am alive. I understand it's wrong, but these thoughts are becoming more and more intrusive, and it scares me.
I am tired of pretending that I'm okay. I want to know if anyone has ever been in a similar state and how you got out of it. How do you stop being just a “shoulder to cry on” for others and start taking care of yourself? How do you find someone to talk to honestly? And how do you deal with these obsessive thoughts?
I understand that I probably need professional help, and I am thinking about it. But right now, I really need some simple understanding and advice from people who might have gone through the same thing. Thank you to everyone who read this.

(English is not my first language, I'm from Russia, so I'm using a translator. Sorry for any mistakes.)