Ok let me explain that SRD are just basic rules. Everything you need to know are in the 3 books. The books are the laws of the campaign you are making. Any thing other than the 3 books only have vague info on everything you need to know. There are more archetypes, classes, sub-classes, even backgrounds to choose from in the books than using SRD.
Now that I clarified The rules, The thing is Once true polymorph is permanent you are now that creature. It says in the books that you become that creature with all of its powers and no longer anything of yours except personality and alignment. Unlike shape change which lets you take the shape of the creature, ''sort of like an avatar of it'' you are now that creature. So once its permanent there is no longer any spell effect over the you you are now it. so once you hit 0 hp you go unconscious or die. if it was still a spell effect you would revert back into your original form, but you don't because you are that creature. Now yes something like wish or gods can turn you back, those things are way to powerful, but a dispel cant work on any creature that has no spell on it. You cant dispel an elemental even though its magical, same point.