A lot of job interviews can sink or swim on the sort of language you use. I'm sure you'll be asked the generic competency questions like 'when have you shown your leadership skills' (and there's oodles of lists of these questions online, so I'd recommend looking them up and writing out some scenarios from your life you can use to answer these), it's best to use assertive language. A big pitfall that many people fall into (including myself, many a time) is to sound too passive?saying 'we' when talking about working in a group, or saying things like 'I think that...' or 'I suppose I did...'. Colloquially speaking, there's nothing wrong with that, but it tends to sound like you're shying away from owning what you did. Making sure you say things like 'I did X' and ensuring the interviewer knows what you did, not what the group did or what you might have sort of done goes a long way. It's certainly not a way most people speak normally, so practice some questions with a friend or relative to try and get that more assertive tone across.
Also, try to structure answers to competency questions: the STAR approach (Situation, Task, Action, Response) is usually a good one to follow. Basically, tell the interviewer what was happening in that scenario, what you needed to do to rectify it/accomplish your goals, what you did (and why), and then what happened afterward. It helps you give concise yet informative answers that don't ramble too much.
Do a little research on the company, too: products/services offered, your job role if you get hired, competitors, previous projects the company's worked on, that sort of thing. A little goes a long way, and it really helps if you get asked the question 'Why do you want to work here?' Also, if you're allowed to ask questions of the interviewer at the end, you can show off that you know a bit more about the company than most other people who want, which is always a plus.
And, to answer all of your questions, I'd say it might be worth trying to focus on the positives. By that (and if you'll forgive me for assuming you have the same type of mind as me, that tends to be very pessimistic), I mean push away the memories of embarrassing or demeaning encounters, thoughts that everyone knows what you did and is judging you for it, and so on. This isn't going to happen in an afternoon, I realise, but try picking out a memory of something you did that made you feel good. It doesn't have to be anything world-shatteringly philanthropic, or even something that benefitted someone else. Ignore the feeling that you're being an arrogant arse for talking yourself up for once, and just let yourself think 'yeah, I did alright'. There's a vicious circle to be caught in by focussing on negatives: you get too harsh on yourself, and then you start to think that not only does everyone know about all those times something went wrong for you, they're also judging you for it as harshly as you are. Getting yourself in a more positive mindset can help break that cycle: you start to realise that most people, if not everyone, have no idea about the sort of negative events in your past (even the ones who were there--human memory is awfully selective like that), and even if they do remember or somehow found out, it would probably be utterly inconsequential to them. I've been there too; learning to not regard yourself as a terrible person is difficult when you have that mindset, but it can--and ought--to be done. You're a much better guy than you'll admit to yourself.
Try adopting things in increments and gradually pushing your mindset away from fully negative. Don't try adopting a persona that you don't feel comfortable wearing, but don't just sit back and let the world flow by because it's hard to change either. Find a reason to be positive, however small (a hobby or a small life achievement, for instance), and try playing off of it when with others. You'll be surprised at how far it might go. Like you said, 'try being yourself' at the moment isn't helpful, but 'yourself' is by no means a static concept. Earn that bit of self-confidence and cherish it.
So, when you go for your interview tomorrow, try thinking 'I'm going to get this job'. Not 'I hope I get this job', or 'What if they don't hire me?', because you're there for that job, and you're going to show them why they should hire you. Talk yourself up, show off your strengths, find something positive in your weaknesses, and don't let that creeping negativity in your mind drag you down, as it's wrong. You'll be surprised at how good you feel after an interview if you push yourself and talk yourself up more than usual. You don't have to maintain it for long, but try making that breakout from your comfort zone semi-permanent and apply that mindset to other areas of your life. It's probably been parroted at you a thousand times, but confidence really is attractive. You don't have to be the guy out of the Dos Equis adverts, but you'll be a lot better off than the guy who assumes the worst from every situation.
Best of luck at the interview tomorrow, and if you feel like it, tell us how you got on. I'm sure you'll do just fine.
