ApeironStella

No worries about the long reply  more detail is better if you go to someone for a diagnosis, anyway.

I'm not going to say it's definitely autism, but I have been through periods that sound quite similar, and I was diagnosed with Asperger's (now apparently a defunct term) when I was 17. I didn't find out until about 10 years later, but I can definitely see that I hyperfixated on certain people and somewhat neglected others in the mean time. One guy I was fixated on (for over a year, maybe two) I walked away from with barely a second thought a few weeks after I "woke up" from it. I also have weird mood swings/fluctuating interest (like, intense interest/happiness, then literal depression/anxiety/low or no interest for a while but wondering why the hell my feelings went away) in most of the people I develop a crush on. But I also get the 'feeling close without really knowing them that well' thing. God, I loathe it!

But I agree it could be autism, ADHD, BPD or hyperfixation (or some kind of combination of any of those). 

@Untamed Heart Thank you! And I considered Asperger's before learning it was a defunct term, so yes. And exactly! Once I feel like I "know" them, "get waked up", I just... have no problem dropping someone like a hot potato and yes, neglecting others is there as well. Only difference is, I don't really get crushes so it was even more confusing for me because when I tried to date them thinking it was a crush, it would just go "????? what wait no that's not right" in my brain. Only second guessing is really for how other side would feel, since I kinda have a huge problem with guilt trips? And I agree/have the same fluctuating interest thing as well! But thing is, sometimes I probably don't even feel that for the person but simply feel it for the topic we were talking about, so that's when it gets messy with "feeling close without really knowing them that well" thingy. Because I do horrible with actual human interactions, so yeah, might be somewhere on autism spec too, maybe?

BPD seems to be least likely at this point since my mood do fluctuate a lot during day but it always match whatever I was thinking/experiencing. So it is not BPD kind of out of control but still pretty changing? And well... There is one psychologist close enough I could find, but I still have this "am I over-exaggerating" sense and since my mother was saying same thing too, not so certain about having a visit rn. (which is funny because this is something I was aware of for... 6-7 years by now? Like, the sense of something is just..off? thingy.)

Aahh...

I think you would probably benefit from going. I know I felt a kind of relief when I found out, because I did have that feeling something was 'off' and I could never put my finger on it, though it seemed like everyone else (i.e. horrible kids at school) could see how I was different. I'm not great at the interaction thing either - if I had millions I'd probably end up living as a semi-recluse or something  

It's still your call at the end of the day, but I wish you luck whatever you decide to do