I know EXACTLY what you mean. I was bullied for years and years at school and it left me totally convinced that I am bad at everything. I say "sorry" all the time for stupid little things because I'm so nervous of upsetting people, so people tell me to stop saying "sorry" as much and then I say sorry for that, and the cycle continues ...
Now, I'd be the last person to praise myself, and if anyone told me I was good at anything I'd blush and deny it, but please please remember this. You are not worthless, everyone does not hate you, and Im sure you are a lovely person. I expect if you look at your basis for thinking you are a bad person, you will have trouble thinking of anything you can put your finger on.Being quiet is not a bad thing. It is sweet, alluring, and gentle. People simply will have spent so much time picking on you that you will have become convinced that there is something wrong with you.
There is not something wrong with you. I very much doubt there was anything much wrong with me either - nothing that merited over ten years of constant abuse anyway. What I WOULD suspect is that you are a caring, slightly quieter and shyer person that the rest of your peers. People see being quiet and shy as "different" and they attack that because it threatens them because they can't understand it. People fear what they don't understand. Maybe they think you are silently thinking you are better than them - and you are, but it's quite apparent that isnt what you were thinking! The average teenage mind cannot cope with the idea of someone who dosent want to comform the social norms and spend all their time getting drunk and being rebellious. They sensed you were probably a more intelligent and caring person and that worried them - maturity isn't something that teenagers have ever valued. They also sensed you wanted something different from what they wanted out of life - and that worried them, because it made them look at themselves and not like what they saw, so they decided to critise you so they could feel better about themselves. Critisism from these people should not matter - they are about as signifcant as mud, and if it is your friends saying these things I suggest you have a word with them about what their "advice" is doing to you. I know all this is easier said than done though.
Please also remember that being a nice, sympathetic, and thoughtful person is misconstrued as a weakness and a bad thing by bullies, and if people have repeatedly put you down it is because they have all sensed you are a nice person - someone who wont retaliate if they get their ego boost by critisising you. Who do you think this makes a better person, them, or you?
Im sure you wont believe me when I say all this, because I wouldnt believe anyone saying nice things about me either, but you seem a nice, intelligent, well balanced and caring individual, and you don't need to change a thing. You'd probably be happier if you liked yourself more, but the first step to that is to stop critising yourself - if other people put you down, don't be a sheep and follow their lead. You are fine how you are. If you accept that, your confidence will grow on its own. It is something I have spent years trying to accept myself and everytime I get a bit closer, I realise that the problem was I was a caring person long before other people my age even knew what that meant. Please don't put yourself down for your positive qualities just because other people don't value them as they should.