Break ups hurt no matter who they are with. It hurts knowing you once trusted that person with so much and now they wont even say hi to you in the street. In my years as a teenager I always believed my friends were there for me no matter what, in years to come we will still spend hours laughing and crying together. Reality hit, that didn't happen at all. I don't speak to anyone from school, and the cruel reality of that is life doesn't always go the way you want it to, and actually people can be really shitty. People fall out of love with each other whether that is friendship or love. It hurts so much when a friend walks out on your life so easily, just like a break up it makes you wonder why you were never good enough. Spending weeks, months even years wondering what you did wrong, just like a relationship you probably won't get the answers. It took me a long time and I am still coming to terms with the fact that maybe I just open myself up to crap people and they just let me down so much that 'breaking up' was the only way.
It has taken me until 21 to realise it is actually okay to stop being friends with someone if they treat you bad, even if you have 10 years of history. People change, some grow together and some grow apart. I feel like I have felt this far too much, I put so much out there that when it comes to it; that person isn't willing to catch me. I am still like this now but I am more wary of people and their intentions. I am forever wondering what people do to make friends for life and why I always seem to not be good enough for that. One day I hope I make a friend who will stick by me through thick and thin as I am not sure I have found that; maybe I never will. Who knows. It is important to remember if someone is treating you bad that you don't ever have to stand for that, you are worth more. I didn't know until recently that a lot more people than you realise go through this.
If you are ever struggling to decide whether it is worth staying friends with someone just think about the last time they put the effort in to see/speak to you, and if it is always a one sided friendship. Then I'd totally recommend walking away. If you do find a good friend who has your back and is there for you through it all, hold on to them with both hands, you do not know how lucky you are.