First, take a deep breath. Then don't forget to let it out again.
From the sound of it, I'm guessing you're in high school or college? Some of it is your age and your hormones. You feel things more acutely right now and it all seems overwhelming. There are studies that show that teenagers have trouble seeing the shades of grey -- that everything is GOOD or BAD, but as you get older you realize that life is just life. It's just a fact. Whether it's good or bad is largely dependent on what you think of it rather than anything it inherently is. IT IS GOING TO GET BETTER. You'll be okay.
(And hey, if you're not a teenager, the rest of this applies anyway.)
As to what you actually asked, there's a quote I heard a couple of months ago that I love in this situation. "What someone else thinks of me is none of my business."
(And yes, if you can master this, you have a massive edge on life!) You have no direct influence over how another person thinks -- and you have no way of ACTUALLY knowing what they're thinking. It's AMAZING how
ASTOUNDINGLY WRONG we can be about what other people think of us.
You do, however, have direct influence over how you think. And you can generally know accurately what you're thinking. So do your best to live up to
your own standards. Everyone makes mistakes. Unless I'm mistaken you're human, and human beings are by definition imperfect.
Forgive yourself as easily as you would someone else.
So let me hit a little of what you said specifically:
- I over-think what i say to ppl now... it makes a conversation and ppl start thinking of me as a jerk.
...
and they consider me as a horrible person.
Okay, you overthink, probably true. But people thinking you're a jerk? How do you know? Are they actually calling you a jerk or a horrible person? If no, then unless you're a telepath, this is purely your imagination. You have no way of knowing if they actually think this. If they're actually saying "man, you're a jerk!" Then you still don't know what they're thinking a lot of the time. People do things for all kinds of crazy reasons. They can say things without thinking, out of habit, or because they're trying to get a certain response from you. People lie, sad to say.
In short, you have absolutely no way of knowing what people think. Obsessing over it isn't necessarily likely to make you any more accurate at guessing. (If you want to be accurate you could try learning about body language, reading psychology of how people think, etc., but all that'll do is make your guesses better, but still give you no way of being RIGHT.)
What you can do is find your own code of ethics, and do things that make you think "I am a good person." Help people when they need it and want it. First do no harm. Tell the truth to everyone, especially to yourself. Don't gossip. Don't cheat. Etc.
And...
I always bring myself down... I noticed that ppl jump at it, and they become more and more comfortable in harassing and making fun of me. So rather than fixing the problem, cus i am worried about what they will say... I LET THEM CONTINUE.
Why
let them continue? Unless you're in a genuine power differential -- if the person harassing you is a teacher or school administrator, why let them? If someone starts in on you, you can always leave. Walking away is sometimes a very useful tactic. No need to stand still for abuse.
As for techniques, it won't hurt to start meditating to bring down general stress levels so that you won't be as reactive and blow up as easily. Journaling could be a useful outlet to let emotions out. And mantras of "I always try to do the right thing" could be helpful. But the most useful thing is to change the way you look at the world. What do you have power over? You. So act upon yourself in ways that will help.
I hope this helps.