Personally, I'm beginning to think that constantly questioning whether you're really an INTP is a legitimate indicator that you are.
It's like: "I have a little heap of data here and the most probable conclusion is INTP buuuuut I could be wrong bc of some bias I have or maybe I'm misunderstanding something, yet at the same time, stereotypes are just that, so maybe I'm overthinking it, I don't have to always perfectly match the general description, right? But also, ..." etc, etc.
And I think that that very king of thought process is a good indicator one is an INTP.
But more seriously, some other good indicators come from the temperament groups, Linda Beren's interaction styles, and some other stuff, that basically comes to this:
Do you use a lot of words? Do the paragraphs flow from your finger tips? Have you trouble getting straight to the point bc you feel you have to really explain it to be properly understood (and maybe to understand it yourself)? (Informative v Direct)
Do you feel uncomfortable walking up to a person you don't know and starting a conversation? Or worse, joining an already-going conversation of people you don't know? (Responding v Initiating)
Do you plan out what you're going to say/do ahead of time, even if it's just seconds beforehand, and feel derailed if something happens in that time that changes what you want/need to say/do? (Outcome v Progression)
Do you prefer to think/talk about "the way things are" (and will presumably continue be) more than "the way this thing is"? (Abstract v Concrete)
Do you think hiding the truth from someone to "spare their feelings" is (at least generally) a horrible idea? In other words, that reality must be faced, whatever we feel about it? (Pragmatic v Affiliative)
Lastly, do you think for yourself? Do you formulate your own thoughts and opinions, and draw your own conclusions, where you can? (Ti Hero, NT)
Hopefully that can help some. Also, I feel all the time that I'm not really smart, that even if I am intelligent it's in the most useless way possible, that I have nothing more than the ability to grasp concepts a little faster than most people. But in the end it's not really fair. I think we're all brilliant in our own stupid ways, and we owe ourselves a little more recognition on that front.
Personally, reading a bit of what you've written, I think the chances are good you're INTP, just from the way you describe your thoughts, though I obviously don't have all the information.
Anyway, yeah, I guess that's that.