Sorry for my poor grammar, English isn't my first language. 

There's a girl I've known ever since we were both in my country's equivalent of 1st grade, let's call her A. While in elementary school, me and A were never BEST friends, but we were friends and we spent time together at school, and sometimes outside of school, but usually through other friends. When we moved up to secondary school, most friends went to a different school than the one me and A went to and we didn't see them as much or as often, so me and A ended up becoming closer friends than before.

 I made friends on my own at the secondary school but A was very shy and quiet and socially awkward (I think she has autism), so it was harder for her to make friends. She couldn't figure out how to start a friendship on her own, so other people always had to be the ones who started it by talking to her. After I had made my own friends, I introduced A to them, and I also helped defend her from bullies and assholes.

My friends liked A, so we stayed friendly throughout secondary school. but as we got older and grew up the things we liked and our personalities changed. She always liked to watch films and cartoons and read books, and she liked 'nerdy' things like Harry Potter and superheroes and animes a lot, and she liked to stay in rather than go out. My friends and me didn't really like things like that when we got older, as we preferred to have more active fun out and about at parties or on day trips, which A didn't like very much because of her shy personality and her self-consciousness. I was seeing less and less of A and now I barely see her once twice a month, and I actually talk to her even less. I asked my friends about the possibility of inviting her out on a trip, or spending time with her doing something she liked or at the very least reaching out to her, but they didn't want to, with a few of them even saying they didn't like her anymore. 

I brushed it off and forgot about it, but recently I did see her again at a restaurant, and she was alone and she looked really sad. I feel like a piece of shit for forgetting how shy she was and how she couldn't make friends on her own, and then I, the one who helped her to make new friends in the first place, took off with the rest of her friends and left her to be lonely. I brought it up to my friends, saying that I wanted to invite her to something we were planning or spend time with her, but they all said that they didn't like her or they didn't want to, and that it was A's own fault that we didn't spend any time together anymore. Am I an asshole for leaving her to be alone? I definitely feel like reaching out for her but would my friends be upset or angry that I was choosing her over them? Am I over thinking all of this? I just feel really shitty for abandoning her like I did.

TL;DR I, along with all of my other friends, stopped talking to my shy, introverted friend and now I'm worried she's sad and lonely. Am I an asshole?