Hello. I read your post. I understand the pain you are going through. My tolerance for other people, socially, is not much higher than zero. i can just cope with one person at a time otherwise i find myself getting lost, standing back and then disconnecting from them. i too am introverted. i spent years of school being bullied daily. Really bad stuff. No-one wanted to be friends with me. Subsequently, i began skipping school, spending entire days hiding from the world in a place that became my secret. I left school with not much to show for it. My story, like many people's is a long one and one i won't go much further with today. But please bear in mind that we are are still here. There is something keeping us here. It might be hope. It might be determination. But whatever it is, there is a reason for it. I am a similar age to you and i know what it feels like to get to a point in your life where you look back and question what its all been for.
Luckily, and I'm glad you brought this up, voluntary work has helped me a lot. I only do a couple of hours a week but it helps me feel connected to the community and feels worthwhile. Keep up with your studies. I'm sure you will do really well. Please feel free to pm me anytime.
P.S. quite ironically i spent 8 years working in mental health services. This can affect anyone. Depression doesn't choose people due to their lifestyles, socio-economic background, hair colour or any other reason. You have done nothing wrong. and actually, you sound like a really good person. i send you my best wishes.