First solo job on my car didn't go so well .... Aaarrrrggghhhh!

Some shops will flow known bad radiators just to get the money. Do you remember if the radiator had any green lookin funk on it anywhere prior to taking it to the shop?
If it did, that was a telltale sign to throw it in the scrap pile. That was corrosion. The reason it's pretty much dead when you SEE green is because they corrode from the inside out, starting at the solder joints. So once you see it, it has progressed a lot. If it was like that, the radiator shop did you an injustice by even trying to fix it. I don't see where any of this was your fault. It sounds to me like you did everything right.