Avoidant attachment style and Avoidant Personality Disorder are two different things. I'm not that familiar with attachment styles, but AvPD and traits I am. She doesn't sound very avoidant (meaning the PD) to me.

Unfortunately, what you're describing, when people have great online chemistry but it doesn't translate as well to real life, that's fairly common. And she ghosted you in a pretty harsh way, but that's also a new common thing. It sucks to no end, and I've been there, and I can relate. It's so hard to worry and care for someone and it turns out they've slowly cooled toward you and can't even be bothered with telling you. Ghosting is cowardly and selfish, and it makes you question everything you know.

I know you're searching for answers, for an explanation, but you'll probably never get one. I wish I could tell you something that would make it better. I suggest you post in the relationships forum, or the coping with emotions forum. Lots more people post there, and they could help you with your grief. The only thing I can really say is, you will get over this, and you will feel that connection with someone else again, and hopefully next time they will feel it as deeply as you do.