I was dx'd very young, and barely remember before it.
But I remember I thought I was bad. That was in... third grade, or maybe second. I came to the conclusion that a "bad kid" would be disliked by everyone and not know they were bad. I didn't know I was bad and I wasn't fitting in, not to mention teachers getting mad at me. Therefore (false syllogism alert), I was bad. (The exact reasoning error is that just because all A are B doesn't imply the converse, that all B are A.)
I remember I knew something was different, but I couldn't say what, and what I tried to figure out just couldn't be, you know? I hated school... but so did everyone else. I was much better at reading than anyone else... but then, I was worse at math. I was terrified of talking to people outside my tiny circle, so I decided I was shy, only for my mom to say things like "oh, she's pretending to be shy." I would protest that I really was, but I had the problem of being talkative and bright sometimes.
I remember knowing I was different when we had to bring report cards home in second grade and the teacher (who usually gave us one language arts assignment and one math assignment) assigned only math homework. Everyone else was elated, feeling they'd had a real reprieve, but I still had 80-90% of the actual work and wished she'd only given us LA, because I didn't actually have it any easier.
I remember knowing I didn't fit in at about the time it started. For a long time, the other kids were oblivious to my oddities and so was I. Granted I had trouble making friends because I couldn't perceive their greetings (not to mention always having a sense of "this doesn't apply to me"), but I was always invited to events and welcome to join in group play at recess. The nadir of my social career up till that point had been when a couple of girls made a mean joke (I think they put chocolate pudding on my jacket and said it was poop) and apologized the next day.
When I got to third grade, I immediately perceived the change. I felt lonely. But my mom dismissed my feelings, telling me I had plenty of friends. Had she said it only a few months before, it would have been true. But the process of being rejected from the group had begun.
What did I think? I thought things before and after getting dx'd. (Possibly because my mom is a rabid curebie who kept insisting she could cure me, and explained to me that I had NLD, which meant half of my brain didn't work, or maybe just because some part of me was scared to be "other," I was in denial for a long time.) I thought I wasn't really human, I thought I was lonely, I thought I was bad.
The scary moment for me was when I had the dx of NLD and wondered suddenly whether I was in any way similar to the girl in the "Dumb Kids' Class" whom I'd befriended because she didn't seem to be in any way abnormal, unlike the ret*d kids (mostly boys) who didn't know how to wipe their noses and spoke with funny accents. (I have never had any patience for stupid people, of any stripe. I judge people by their intelligence.) I wonder to this day why she was in that class.
As a kid, I invented a language just to write a heavy-handed allegory where my mother became supreme dictator of the world and disagreeing with her was pathologized and outlawed. Naturally, I (I wasn't even trying with character creation) overthrew her regime and ruled benevolently with the organization called AINAP, which was an acronym I secretly came up with to describe the spectrum after being lumped in with them and deciding I may as well include them in my secret defiance. (Stood for "Autism Is Not A Problem." Even as a kid, I understood that much. Alas, there's not the same gut reaction against "you have mercury toxicity and we can fix that," so I came out believing that Autism Is Not A Problem, but that it could be cured. I was one scared little kid for a while there.)
Tangent aside, I thought I was some undefined "other" and maybe bad. I kept thinking I was bad (to the point of almost being delusional) for some time after, but I fixed my self-esteem issues a couple years ago. But this doesn't even get into how it feels to be proud of "a disability" and to want to keep something you're supposed to be curing.