Is it normal that i hate the thing my mom said?

First of all, calm down a bit.

I believe that as you get older it becomes harder to make new important friendships, and that maturity in a lot of ways is the same as not caring so much.

I agree with you that you should not just let things go, if they matter to you. I don't know why you can't reconnect with your friends, but if you can win back something that is dear to you, then go for it.

The last part of your post comes across more as immature than youthful, and is idealistic in a bad way.
You don't want any friends, if you can't have your old friends, and you feel that acquiring new friendships would be a betrayal?
If you were calmly stating this as factual circumstances in your life, then I could respect, but with the way you express yourself, I would rather advise you to be mature.

You're 19, and you stated that you're not mentally an adult, so you still have years of development. In the coming years there will most likely be people that will mean a lot to you, whether they are your old friends or people you haven't met yet.

Some friendships come and go, sometimes people grow apart, people lose touch. It may be sad, but that's the way things go. If you avoid that in your life, then kudos to you.

On the point of it being something your mom said, consider the case of it having been said by some random person, whom you had asked for advice from. How would you then react?

Maybe your mom doesn't understand. Maybe she can remember her own youth and does understand, but is giving you advice that you would have given too, had you been in her shoes.
Maybe you are not viewing her as you would someone else, misconstruing her opinions as something that you oppose.
Maybe she hasn't given that much attention to your problems, and considered what matters to you.

Yeah. Your brain isn't fully developed until your mid twenties. You still have time before you are fully capable of growing up.
One of the key aspects of "growing up," is that you learn how to control your feelings, think about things in better ways, and how not to sweat the small stuff. Your mom is right in saying that if you were to "grow up" or "be mature" then you wouldn't be worrying about such trivial things.

Friendships really don't last forever. To be 19 and still not understand this, you must be very lucky.

I'm 20 and what I've noticed is that after I graduated high school, I lost about 90% of my friends. It's pretty difficult to maintain old friendships after you become an adult with few exceptions (ie, going to the same school, living closeby, etc). If you don't have any of those exceptions, you might as well just give up. Your mom has a point. You can always make new friends and "act properly" with them.

I refuse to read this all. Missing out on something?! Oops, my bad, I'm sorry!

You come off as being incredibly immature Und unreasonable. Your mom is/was giving you some nice advice but yet you flip out which further proves that you need to grow up. Sure, no one wants to lose their friends or grow apart from them but that doesn't mean that you can't tighten the relationship by being mature. Heck! I'd be more respectful towards someone with maturity than someone without it. You can't avoid your responsibilities and life forever.

Your mom doesn't have any advice for this one. Looks like you'll have to let those words go, be mature and find your own solution.

Oh quit complaining! Whiny little brat.

*adjusts his old man hat*

I have gas... and Ben & Jerry's Strawberry Cheesecake ice cream flavored acid reflux. Mmmm...

yeah thats right

My eyes they burn.

I just bit my tounge.