Wondering if I'm always unpopular because I'm autistic?

I would seek out like minded people. Don’t feel bad that friendships haven’t worked out, they’re just not your people. You need friendships with other autistic people and/or people who share interests with you.
Dont rule out assessment and diagnosis. Whilst you don’t think it’s of any advantage work wise, it cou”d be really helpful to help you understand yourself and feel better about yourself.
What are your interests btw?

Find some autistic/adhd people and see if you fit with them.

Happened to me.

go see a therapist. I’ve been going for years. Always lonely, never feel included. I’ve got a great job, loving kids and a fiance. Not many friends. Like, 3 max.

Try and make friends here and there and have individual friendships don't try and merge with a group. Often all the others are more close because they've been friends longer. They you can feel left out, even if they don't mean it.

I agree with @MintiLamb and @Comingupriver

If you seek assessment, you might get a diagnosis, or you might not, but either way, you don't need to tell other people if you choose not to.
However, at least looking in to things a bit more, might help you understand yourself a bit better.
It might also be easier if your close friends know, so they don't think you are being obtuse, but understand that you genuinely don't understand a lot of the nuances of language and the social cues that most people pick up naturally.

I don’t normally post about these things but I will for balance…

I am autistic. Diagnosed as such 11 years. Now in my late 30s

I have always had friends. I’m one of a group of 5 best friends (they’re neuro
normal). I am definitely the odd one out. And I take things very literally which is confusing for some people. I do not understand subtext or vagueness. I’m also very naive and have sometimes needed rescuing from dangerous situations which I hadn’t noticed 😳 especially when younger! (I’m very risk averse these days!)

I’m usually first to go home because I’m sensory overloaded and my ears are dancing & my brain is like a radio trying to tune into a far away FM station.

DS is autistic too and thankfully all my friends, their children and their husbands are all really supportive and patient with me. I’m very lucky.

Im very good at masking, but it’s just so tiresome!

I'm probably autistic, have read into it very thoroughly and found strategies recommended for autistic people helpful. I'm not pursuing a formal diagnosis atm (other health issues) but feel better within myself having "realised".

I struggle with groups and "fitting in" so mostly just don't chase that any more, I am friends with individuals.

I come across as really friendly, but after a short while people move away from me.

Yes. People seem to only like me in small doses and for a short while.

I had years of people asking me if I'd considered I might be autistic before I was diagnosed. I didn't really know anything about it the first time it happened.

I won't pretend some of those weren't being dicks but most of them were well meaning.

I very rarely tell anyone.

But, yes, I have come to the conclusion that it is largely why I am unpopular.

I have a friend who may or may not be autistic, but she does need everything in a conversation explaining to her as you describe, because she often seems a bit puzzled by what people are saying - like she doesn't pick up on any nuance or subtext. Does that sound like how you feel?

I think a diagnosis would help you understand yourself and therefore work on stuff or learn to accept it.

Those people sound unhelpful. It is fine to ask questions.

What do you mean by: 'I'm not going to pursue a diagnosis because frankly it's no one else's business.' You would have to share any diagnosis if you didn't want to, but it may help you understand yourself a bit better.

Sorry you're feeling lonely. Feeling excluded is really hard.