long story short: She's toast.

But if I may create this near useless thread out of sheer curiosity of learning more about this transmission from real experience, I'm less interested in the actual diagnosis in the classic sense of trying to fix it, I know she's gone (or at least I presume as mechanical laymen) - Im just curious what you guys think actually happened to this ol' girl.

99 chevy silverado, I even just assume this is the transmission in this truck, I could be wrong(?) - Bought the truck a few months ago, shifted nice, pretty firm, 1st gear pulled like a bat outta hell and shifted real firm to 2nd- I took it to the Silver lake sand dunes up here in Michigan the past weekend and accidentally overheated it. Getting to a parking spot to cool off she was touching the red/orange zone. Got trans fluid hot message, barely made it home slipping so bad but now here are her final symptoms that I am just curious what may actually be happening inside it.

My gear selector will not go 1st manual and I don't know if it's designed that way or not since I bought this truck.

In D, slips so bad can barely get it moving (doesn't shift just revs and moves a little like its a higher gear?)
Reverse works good
3rd manual is literally just 1st gear stuck, still pulls like normal I have to manually select 2nd for it to shift to 2nd so I can at least cruise past 20ish mph lol

thanks for reading if this post is retarded feel free to delete i just like learning about these types of things and figured a forum is a good place I often come to threads from google about various chevy stuff.