What's wrong with me?

Okay, so I used to be absolutely fine a few years ago. I loved talking to people, I felt confident, cracked a joke whenever I liked and didn't feel upset If people didn't laugh - I'd just make another one later and people usually found it funny. I would spend my evenings talking to friends and just enjoying life, but then there was just a random point where it stopped. I don't know what happened, I just didn't want to speak to people and every time I went onto MSN (that was where I spoke to my friends) I ended up appearing offline as soon as people came on and I just switched to Youtube. Now, I get nervous when talking to new people, ignore messages because I'm scared of conversations starting, and when I get invited out to places by my friends I don't want to go because I think everything over like 'What if they get bored of me? What if it gets awkward and we have nothing to do?' At first, I didn't care that much because it felt like that was just me, but then ever since my sister went to university it feels like stuff is getting on top of me.

And then there was a moment that made me break down in tears. I miss my sister so much, but when she skyped me, I sat there still and shocked and signed out. I don't know why I did this, it felt like it would be an awkward conversation but she's FAMILY, I just don't get how I could feel like that with someone so close to me. I'm terrified because I keep thinking, what will happen when I go to university? Am I going to end up not talking to my family?

When I do actually answer people's messages, or when I've been talking to someone for a while in real life it just feels like my energy goes. For example, in lessons at school, if I sit next to a friend then for the first few minutes I'll be energetic, but eventually it will feel like it drains every last drop of my energy when I keep speaking.

PLEASE PLEASE PLEEEAAASE HELP ME

I just don't know how much longer I can handle being like this, I'm missing out on so many opportunities and friendships just because of my awkwardness