I don’t understand why I’m losing friends? I’m 19 and it seems like all the people hung up with just stop contacting me or go separate ways

At least I have my four best friends I’ve known since childhood. A couple of friends we’ve had, no we just don’t talk anymore and some just stop contacting us. I remember my mom saying that one of her best friends she’s known since childhood got super popular, hung out with popular people call and stop talking to her. They haven’t spoken since there were 14 (1988) that’s been like 33 years! We said that that’s never gonna happen with my best friends and I and it still didn’t. My mom said unfortunately that happens sometimes and as we get older we’re gonna lose friends whether we like it or not. There is one girl that hung out with my parents trying to warn me that she’s not my friend is she gives me nasty looks behind my back and I didn’t wanna believe them. They tell me to be careful when I said she’s not my friend and I was like no that she is. But unfortunately they were right and she was not my friend and she was nasty to me behind my back. I HATE when parents are right. I don’t wanna be the person with no friends or just the one friend but I learned it’s better to have a few two friends then like 100 fake ones! I think it’s sad that we lose friends as we get older it seems like we had a lot more friends when we were younger than we were older. I wasn’t mean I’m like some people I will talk to they just stop talking to me and then when I talk someday just don’t reply anymore.