Hi there,
Totally get why one would get angry with themselves because they know in another context the issue does not exist. I think you're valid in feeling frustrated because of this.
I find I'm exactly the same. In a context where I'm feeling safe and comfortable I'm super outgoing and dare I say charismatic. I see this is the case for you at school. However, in a different context where you're not feeling safe (in terms of socially secure) and actually uncomfortable, the conditions to be outgoing are not there. So it makes sense that we do not act the same. I get like this at drive thru's/speaking with customer service anyone. I stutter, I panic, I just can not hold it together. It would stress me out so much because I didn't know about the context thing I just told you.
My therapist and I have been doing exposure therapy throughout the last year I'd say. It's been so helpful. At first I absolutely hated it. Just like you I got extremely anxious about what I've said (and how I've said it, and how I'm an adult panicking at a drive thru because I don't remember my order and I'm taking too long etc etc). I'd too beat myself up afterwards for looking stupid etc. It was life changing when my therapist told me that thoughts aren't always true. Like, there are times when my thoughts are me, other times, my thoughts are anxiety. Anxiety isn't always telling the truth. In fact, anxiety often lies. For example: "You didn't say anything in that conversation they think you're boring". But there is no way to know how they're thinking because you're not in their head listening to their thoughts. It can seem VERY true until you understand what kinds of things anxiety likes to say. Then you can see when anxiety talks and throw it away essentially.
I would say the outgoing carefree persona is not something one loses. It's something that comes out when we feel safe and comfortable. But sometimes we have to kind of force ourselves to feel safe and comfortable in certain contexts. To do that, start with lower stakes social interactions that are on the lower end of anxiety. So for example, 1 being tiny, negligible anxiety. 10 being absolutely not - this will be the end of me. This, of course is different for everyone, but it could be waving to somebody you know at the shops or smiling at a stranger at the shops or something. Continue to do this until you feel the anxiety 'grade' you've given it lowers to a manageable level. Then continue to up the ante.
For me, I still get nervous at some drive thru's if I'm having a bad day but it's definitely not at the level that it was. I've done it so much over the last year that it's so much easier for me.
I will caveat though and let you know leaving high school is a BIG social shock because the way you made friend's up to this point is not the way you make friends now. HEAPS (if not most) people feel super lonely after leaving high school. Especially if you're going into a whole new space like uni, TAFE, or full time work. That is something I'm telling you now most people experience. But continue to practice being aware of when anxiety is talking (I used to write down the common things my anxiety would say to make it easier), and practice exposing yourself to social situations that makes your social anxiety shoot up on an incremental basis. Your confidence will naturally build with experience.