I get really bad social anxiety

Like to the point where it's almost a fear in some situations. Last week I joined this strongman bootcamp with me older brother and for days beforehand I could not sleep or eat. I had so much anxiety, I could barely even workout the days before

Once the session was over (it only lasted an hour) I was feeling fine again.
It flares up even when I'm just meeting up with some old friends. I'll just start regretting having said yes and looking for excuses.
I know it sounds stupid but I think I'm more scared of rejection. Like if I say something stupid or if I'm not as good as the other people.
I cope with it by reminding myself of the reality of the situation. Like say for an interview, I just remind myself of how great I am and why they should employ me

It's worked nearly 100% of the time and I have been offered a job to nearly all the interviews I've been to. Or say like with the strongman thing I attended. I just reminded myself of how good it will be for me and how everyone had to start somewhere. Of course I was the worst person there

But I intend in going again tomorrow. Or when I've been invited out, I remind myself that I will only regret not going out if I drop out. Once I actually leave the house and meet up with everyone I forget all about it and I'm fine again.
I should really get it diagnosed by a doctor sometime and see if they can help but that's how I have learnt to deal with it.