I would guesstimate that the majority of writers consider themselves shy or introverts. So, first, the best news: you are not alone.
The second best news: platform development is a process. Remember, nothing happens overnight. Therefore if you are shy or an introvert, this will not likely disappear all at once.
What's more likely is that you will slowly become more brave and outgoing.
I was painfully shy as a child. My mother started doing daycare in our home when I was a toddler because I was too shy to leave her side. So I’m not likely to see shyness at the end of the road. I view it as a starting point, not an ending point.
Like any speaker, when I got started, I had zero experience with public speaking at one time. Nobody is born ready for public speaking. Speaking skills are always learned—sometimes through training, sometimes through experience, most often from both.
I outline all the steps that writes can follow to become comfortable speaking in Get Known Before the Book Deal. The gist of it is start with what’s small and manageable enough for you that you won’t chicken out. Then grow the size of your audience gradually over time. This is what I did. I started with groups of three to twelve. Then grew to groups of twenty to fifty. Now I can manage two or three hundred people as long as I have ample time for preparation.
It’s human to resist what’s uncomfortable, so don't be too hard on yourself for not wanting to come out of your shell. However, speaking is something authors do. So I’m not likely to let you wriggle out of it.
If I could overcome my painful shyness, then anyone can.