I’m an introvert, too, so I understand! It can be intimidating, particularly when we are young.
Obviously, as TTLE said, praying for the Holy Spirit’s guidance and for a sense of peace is important to do. He wants to help you!
I think something else that helps to overcome that fear is to go to Confession frequently—generally, once a month. The more often we do something, the more familiar it becomes, and it becomes a little easier and not so scary every time we go.
As for what to do at this
next Confession of yours, I know some people find it helpful to write things down. That way, once you get in there, you don’t have to worry about forgetting anything, nor even concern yourself so much with working up the nerve to say this or that sin. All you have to do is read your little paper. That always helps me in stressful situations. I don’t have to use my brain as much, nor worry about articulating things on the fly. All I have to do is read what I wrote.
Of course, you want to make sure you destroy the paper afterwards.
I will say a prayer for you regarding this and your upcoming Confirmation. There are lots of graces in store for you! That is awesome!