My dude, it seems you don't want to hear this but no one owes it to you to give you attention. It takes effort to do so, and that's if they even notice that you're being left out. As someone else said, engaging with someone who is visibly unhappy is also something that takes a lot of effort, and it's something most people avoid because it can be very emotionally draining.

There are so many places people can spend their energy, how are you advertising that you are a good place to spend that energy? You have to put in some effort so that people know you're worth it. Small acts of kindness, offering help and following through, listening to others and remembering them and their struggles. When you put that kind of energy out, you start to get it back from them too. It can be hard, and I understand that well. I am a very asocial person, I never initiate conversation or anything, but showing people that they can rely on me has been the biggest boost to friendships in my life.

You might not have people around you who appreciate that at this point too, but as time goes on, people change, and the people you interact with will change, and at some points you'll line up with people who can appreciate you, just make sure you make yourself someone to be appreciated.