Heres a small piece of advice.
Best friends, no matter how good their intentions, and no matter how much they don't want to hurt you, still will. They'll tell you about something nasty a person has said about you, or that someone doesn't like you, and even though they tell you so that you don't find out from someone else later and get hurt twice as much, it still kills you inside to hear it.
When people say they're sorry, you usually mean it, and when you're forgiven you think everything will be okay, but some mistakes can't be forgotten. Some mistakes will haunt you for the rest of your life, and when you think everything is better you find out that its not, and that mistake will come back and bite you. Just keep reminding yourself, you are a better person for apologising, you can't take back your actions but you can try your hardest to make things better.
Everyone needs to learn that not everyone has the purest of intentions. When someone says they'll always be there for you, that they'll never hurt you, they usually do, doesn't matter if they intended to or not, someway or another, they will. You'll be left heart broken, and confused, and even when you think you're over it something that person says or does will trigger you heart and make you remember how you used to be able to trust that person and now you can't. What you have to do is remember, don't trust to quickly, its not hard to lie and if someones good at it, they will, so figure out who you truly can trust, and you may get it wrong a few times but you'll figure it out eventually.
When someone you loved leaves you, you will always feel hurt, no matter what, it will get to you, whether you show it or not, and when you are finally over them, if you really did care about them, finding out they've found someone new will still hurt. You may question yourself. Ask yourself what they have that you didn't, is that person better looking? Is that person fitter than you? all things you may ask yourself, but all you have to do is remember you are beautiful, no matter what and that person doesn't know what they're missing out on. (no one believes it when they hear it, but its nice to think about).
Even after all of this you have to remember we are taught through life that things happen for a reason. That no matter how bad the situation and how crap you feel right now, something good will come out of all of this. We are taught this from a young age to help us through all the struggles we may encounter, there are some things you will encounter that you won't know how to deal with on your own, but we get told to remember that something good will come out of it.
People feel worthless everyday, you may not notice, they may be the most bubbly person you've ever met, but things will get to people, even if its a teacher in class being hard on them, or a bully in class who called them fat, or their parents are fighting at home, or they where sexually assaulted, you don't know whats happened in their lives, its not hard to pretend, so be careful about what you say, your intentions may be pure, but they could serious consequences.