I have learned the hard way that letting other people influence you can only lead to trouble. It is one thing to change your beliefs as new information comes in, refining the good and removing the bad, but it's a different issue to let your brain be used like the play dough in a day care. You simply should not let just anyone stick their hands in and mold who you are becoming, you have to pick and choose and ultimately mold yourself. Unfortunately, I have cultivated over time less and less patience with easily influenced people, because they always end up turning on you at the drop of a hat just because 'so and so' said something against you, and it makes no difference if it was actually true or not.
I've been through a few trials and tribulations in my life, but if there is one thing that it has taught me, it is that life is what you make of it. Nobody can make you do/feel something unless somewhere inside you on some level consents to letting them. The only person in control of yourself is ultimately yourself, with lots of emphasis on introspection and self restraint. I am not trying to make it sound like that is an easy thing, because it most certainly is not, but the effort is what is important, it starts the journey to becoming that awesome person that you strive to be.
It's difficult at first, but you should not care too much about what other people think of you, because there will always be someone who wants what you have (or thinks that you have) and there will always be people who are more focused on your life than theirs because they are too miserable to look at, much less fix, their own problems.
Usually the people who are calling you names and spreading vicious rumors about you behind your back are the same people who are taking advantage of you and are friends to your face. I've never understood that pretense, as if it's somehow necessary to not be a real person, a human being, in front of other people to survive.
I think that some people become that way because they have been unhappy for so long that it feels like there has never been any other emotion, so that depression and misery turn into callousness and spiteful envy which become their normal functioning feelings. This doesn't happen out of the blue, people who act out in such horrible ways are typically victims themselves of various childhood abuses/neglect or traumatic experiences. That is not an excuse for such behaviour by any means, but the abused and scared simply have a higher possibility of becoming abusers unless they consciously break the cycle and decide that they refuse to harm other people (be it physically or mentally).
For some of those same people, those needs to be accepted, loved, cared about, and feel good are so great that they go out of their way to please other people and bad mouth whomever is the object of their dissatisfaction just so they look better by comparison, and all that does is hurt everyone in the process. Deep down, they are probably a wonderful and well meaning person, but the fact is that their actions reflect someone who has been hurt over and over again, and is now hurting someone else or themselves because that is all they know how to do.
It really doesn't matter, even if you're not that 'bad guy' or that spiteful person. We all have things that we leave undealt with over time that cause us grief and distress, many times without us even realizing that it's an underlying problem coloring our actions, reactions and thought processes. The only way to have lasting happiness is to accept who you are and make peace with the pain that has been forced on you. We do not get to choose who hurts us, we only get to choose how we react to it. You didn't want it, you didn't ask for it, but you're stuck with it, so it needs to be handled.
Only then can you make someone else really happy. Fixing yourself absolutely must begin before trying to have a successful relationship, romantic or friendship or otherwise, as without that it is almost certainly eventually doomed. (Notice that I said 'must begin before', not 'needs to be done before', because that's probably an unrealistic goal.)
We only have control over ourselves, no one else, so doesn't it make sense to become the best you that you can be before trying to find another person to spend your life with or trying to be 'friends' with someone that you barely know? Making lasting connections, making friends for life, and finding a life partner is easier and more fulfilling when you are already happy and not relying on them to 'complete you'. Emotional baggage doesn't need to be permanent and those old scars do not have to mark you up forever as 'damaged goods', either.
We all deal with and react to things like this differently, there is no one-size-fits all. It's going to be a battle, but it will ALWAYS be one worth fighting. The first step is really just looking in the figurative mirror and asking yourself, 'why am I like this?' and then coming to terms with all that resurfaces with understanding and forgiveness, one painful step at a time.
I only speak from acute observations and personal experiences, especially in recent light. Sorry for the rabbit trail :P