When I do unknown or out of comfort zone things (and my comfort zone is quite small), I'll get butterflies in stomach every time I think about it, and I'd procrastinate from doing it. I've realised that this anxiety is a conditioned response, because it does not help the situation one bit, and I can't rationally think of a reason that I should get butterflies in my stomach even thinking about the thing. But nonetheless my body just tenses up by itself.
I don't think I've gotten many events as serious to be called 'panic attacks', but once I fell into an anxiety spiral trying to sleep. I wasn't falling asleep, which made me think why, which made me think that I couldn't fall asleep because I was anxious, which got my heart rate up, which made me more anxious, which made me think I wouldn't fall asleep. This cycle continued until my heart rate was so high I had to go to the doctor because I thought I was going to die.
I've found having these experiences actually helped me to control future experiences from spiraling out of control. For example, I knew if I let myself spiral out of control I'd get a panic attack, so I'd try to distract myself to not let myself spiral. I also know that the anxiety will correct itself, e.g. if I do actually get a panic attack, I've made it through and I won't actually die, the body will tire itself out eventually.