Whenever a guy likes me a get really nervous

I’m 17 - turning 18 soon. I’m not sure why but whenever a guy likes me I get really scared and nervous. I’m definitely an academic girl, I’m achieving all A*s in my alevels, and I’ve always made that an excuse as to why I don’t hang out with guys outside of school (because I’d rather focus on my education). Though I talk to some guys on text.

This situation has happened multiple times and I don’t know why.
The first time it happened was when I was in year11. My best friend of 4 years started flirting with me and all his friends were saying he liked me and he asked me out on a date. I got so nervous that I started to ignore him because I got scared almost? I really did like him back (I still do) but I literally wouldn’t talk to him on text, in person for multiple days and I wouldn’t make eye contact with him.
I still avoid eye contact with him now. He’s got a girlfriend and I’m not sure if he’s moved on 100% from me because you know when you have a little feeling that there’s something still there?? That’s how I feel.

Anyway, I sit next to this really smart ass guy in maths and he’s so lovely to me. We’ve got loads in common and he helps me in maths without making me feel stupid and he’s great. I don’t like him as much as the first guy though. If I had to choose between them, I’d choose the first guy any day but obviously I can’t now.
So me and the guy from maths have been talking for a few weeks now, and my friends thinks he’s been flirting with me.
When I was tired and wanted to end the conversation, he kept it going by asking me questions and somehow his friends thinks he likes me too as they call his name from across the room when I’m there lol.
I’m getting that same feeling and I don’t know why. It’s like I want to ignore him and I just get so nervous going into maths to sit next to him. Why can’t I just relax like a normal person??