It does seem like you've got some social anxiety going on. I kind of struggle with this myself, and have since I was a kid. So I understand how you feel. Your mum should let you get some counseling. In the meantime, here are two things you can try when you don't know what to say to someone:
1. Find something to compliment them about, and be sincere. People love to be complimented, and will like you for it.
2. Ask questions about them. What's your next class? What do you think of the book we're assigned to read? What are you doing this weekend? People love to talk about themselves and their opinions, so they'll like you if you give them the opportunity by asking them questions.
You might try reading the book "How to Win Friends And Influence People" by Dale Carnegie. It's a classic that's been around a long time and has helped many people. The audiobook is on YouTube.
I hope that helps!