What is a friend?

I don't understand what friends are. Someone I thought was my friend recently started feeling like they aren't. I can't talk to them about my problems. I can't speak my true feelings. It's awkward to talk. They look at me with cold eyes. Are they really my friend? This doesn't feel like friendship. But they used to be. We could tell each other anything, and we'd talk for ages in front of my house after school. We got through a lot together back then, and we even went to the pool together. They used to be quite violent. I chose to endure it. But eventually, I couldn't take it anymore, and after not interacting for about a year, they came back having changed. Now, they aren't my friend. I recently realized I'm quite a jealous person. Originally, I was the one they talked to the most, and we were the closest, but the situation changed. Now, my friend talks to people from their club, and I hate it when they look my way and pretend not to see me. It's incredibly selfish, but this is my true feeling. However, when we talk normally, it feels like we're back to how we were. I feel sorry for myself for getting so angry and almost hating them behind their back. But then I think again. I'm really selfish and have a bad personality. What is a true friend? If this is what friendship feels like, then I don't need friends.