Hi there Omelleteee!
I think that what you are going through is perfectly normal. It’s okay to feel left out and jealous of other people, the dividing difference between ‘good’ and ‘bad’ is what you do with those feelings. What your friend did was in my opinion a bit inconsiderate, but everyone has their days.
I had the same problem as you, I would make friends with people, only to realise they didn’t really think of me in the same way. A good piece of advice from my parents was,
“Friends are here and now, friends are people in the present who make you feel happy and who you make feel happy. Friends are not forever sadly, as people change over time, all you can do is keep with the friends that want to stay and let go of the ones that don’t.”
At the moment, my best friend is kinda, leaving me behind too. I don’t have social media and she lives very far away and when I visited her she had changed so much, she wasn’t the person I had befriended. That’s a prime example of someone changing and the stable relationship slowly fading.
Social anxiety can be tough when you’re trying to make new friends, so I understand how a close friend leaving can affect how you feel a lot more than one of mine leaving. But, Your a hundred percent qualified to be someone’s friend, maybe just not theirs.
Now you could always wait for someone to come over and befriend you, but sometimes it doesn’t work very well, so every now and then, if you feel up to it, sit next to a new person or add a few comments to a conversation. That way it shows you interested and open to conversing and associating with them.
I wouldn’t be to angry at your friend, either. She is just changing and moving on.
Also by the way, dressing casual is super comfy and super cool right now! So not only is it practical, but where I am, it’s good fashion. I have to say I live in pjs and shorts paired with literally anything.
have a nice day!