Social Anxiety / Not trusting people

[Kitten_]

This is kind of hard to explain but I'll try my best
I always assume that people who talk to me only talk to me because A) They're playing a joke on me B) they're making fun of me C) they feel sorry for me
I assume this until I get to know them better and we start talking more and more and then I realize that they GENUINELY like me (boy or girl). It's driving me insane because I'm so suspicious if a guy comes up and talks to me because I (for whatever reason) think he's making fun of me in his head? I have no idea why I think this and it doesn't help my social anxiety at all. I already have a hard time socially and then feeling like someone's only talking to me to make fun of me makes everything worse.
I have never been bullied, I've never even been made fun of. I'm a really good track athlete so I'm very fit/slim, in all honesty I am quite pretty/cute (based off of my thoughts and what others have told me) and I dress really cute. I catch eyes with guys a lot, without meaning to, and every time after I catch eyes with a cute guy I convince myself that they were only looking at me because they think I'm an idiot.
There is this really nice guy at my school who used to be in my biology class. He was in my group for a lab and he was really funny and nice to me, unfortunately he dropped down and he's not in there anymore /: But he goes to track meets and etc. and every time he sees me he always says hi. And he gives me high fives and like today I gave him a super awkward one and he was like, "hey, (name) you know you have to do better than that!" Anyways, although he's very friendly and etc. I've convinced myself that the only reason he talks to me is because he thinks I'm a shy idiot. I know logically that he doesn't think that but I feel like he does and it doesn't help my social anxiety and low self esteem AT ALL.
How do I start to correct this? It's extremely irritating. Mostly it's around guys, although I feel the same way around girls who I don't know at all sometimes. The thing about guys talking to me is that I'm very shy and never start conversations with guys because I'm afraid of annoying them, so if they start talking to me it's because they want to, but my brain says its because they're making fun of me.
In addition, I can NEVER start a conversation with a guy. PERIOD. ever. Because I'm scared that he only acts like he likes me and secretly thinks I'm stupid.

Sorry for the novel /:

[Miserabilia]

Well like you said you obviously have some trust issues, rather than social anxiety. (I'm not ruling it out though)

Do you have any good friendships?
If you have a deep friendship you can learn to build your trust on them.

[Kitten_]

I have one really really good friend. Sometimes I convince myself that she secretly doesn't like me but I KNOW that isn't true because she's always asking if I can come over and you can tell she feels more comfortable around me than her other friends… we talk about EVERYTHING haha. And when I'm around her my social anxiety actually gets a lot better, she gives me confidence… but, of course, she goes to a different high school. /:

I guess I probably do. I've never been hurt by anyone that way before, idk why I have such a hard time trusting that people actually genuinely like me.

[JollyToes]

It's not an easy thing to explain or pinpoint a certain event that causes this. It sounds like you really have a good friend and she is someone that you can trust. Maybe you could invite her along with a couple of people you know from your school to hang out. With her there it might make things easier on you and less stressful.

[CcRoder]

OP, there's a psychologist at my hospital who used to explain that people who were introverted and had the General Anxiety symptoms were more prone to what you describe. So like, sometimes any person with GAD can have a problem feeling empathy, or trusting someone. IDK what causes yours, though the poster before me has good advice.