I have noticed, quite recently, all the way through my life until about 2 years ago I had always strived to make other people happy, which made me timid and quite pathetic, but after I got new friends I started going with what I wanted, not other people, which was good at the time, I had made a leap in personal growth.
But now I'm too selfish and possessive, and that's made me insecure, which makes me more possessive. You see where I'm going with this?
I'm terrified of my friends suddenly turning around and saying they've had enough of my selfishness and hate me, I think I'd kill myself, or at least try if that happened.
Especially with this one friend (he can be Sam); He joint our group last year, I had hated him in middle school, but now I love him to pieces (not romantically) however, I've started getting really pissed off with another of my besties (lets call her Amy) because she's talking to him more than she used to, which means he's talking to me less. And even though I love her and she loves me, she told me she loves him more; I suddenly became very afraid at this point, and now Im terrified she'll take him away from me and I'll be alone (because recently my now ex-best friend had taken up smoking and just dropped me)
My friends and I always joke on about how I treat 'Sam' like a slave; I don't really, I'll ask him to do things like pick up a pen I dropped or something, and I give him money to buy me lunch because Im repulsed by the Idea of getting it myself. (weird right?) But 'Amy' has started getting at me for doing that, she'll make a few remarks I can brush off because I know her, but now 'Sam' is starting to do it too, and sometimes they'll have a conversation and I'll try to join in because I basically know what its about and everyone usually wants to talk to me (I'm not being full of myself here) but sometimes 'Sam' will look at me and say "Sorry! You can't be part of this conversation!" and he doesn't realise how angry, depressed and sick that makes me feel, I can't tell him because I'm just being selfish and don't want to share, but it's starting to eat away at me, so I want to learn how to stop being selfish.
Any tips?:(

You probably have your defense mechanism on not wanting to be used again.
You need to make a conscious effort to stop and think, 'what do I want, how is it effecting others, what would they want, what is a compromise and win/win answer'.
Don't just run with your feelings and emotions.

I haven't said anything to either of them yet, that's reasoning with myself.
However I think If I don't get rid of the jealousy I think I might end up hating 'Amy' and I really don't want that to happen because we're so close.

Remind yourself of the pro qualities that you like having her as a friend and be a friend.
Realize what issues are yours and separate it from her problems. In other words you need to learn to stop projecting your ill feelings onto it being her. You may think you have but as long as you are having attitude you haven't.

Yeah... I'm really trying, but it's really hard because in our group, one person's problem is everyone else's, I won't let anyone get away without telling me what's wrong and they do the same.
But just now I spoke to 'Amy' and I feel so much better, because she told me that she felt bad after saying it and that the only reason she liked Sam more was because he's male.
But I'm still wary of my selfishness and possesiveness, I wonder if it would end after school? When I'm no longer around them every day?

If you want to take more drastic mesures to change, you could start with a plan.

Every day, do at least 5 good deeds for someone else (that doesn't benefit you). Eventually, you will stop counting and it will happen naturally.

Ok, I'll try...