Extending the Free Speech Clause to all private organizations is so goddamn ridiculous I don't even know where to begin...

if Twitter won't let me on their platform I am cut off from political discourse

No, you're not. Get off your computer and go outside. There's political discourse happening in the real world, too.

because corporations are becoming so powerful that we can't function in society without them

Have you stopped to think that maybe the reason you think corporations are so powerful is because you are exclusively relying on them to communicate with the outside world?

These corporations wouldn't seem so powerful if people like you stopped living in a digital world, got off your ass, and actually did some work in the real world.