You describe exactly how I felt this time last year, OP. Like you, I was preparing myself to fail my final exams and was almost disappointed when I passed because I just did not feel ready to start working. For various reasons, I always felt less confident than the other students in my year, so my entire summer was spent worrying about how I would cope and thinking that I just wasn't good enough to be a doctor. It has taken me a whole year and a lot of positive feedback and reassurance from seniors to realise that I am doing OK and do deserve to be here.
My biggest advice for you at this point would be to try and increase your confidence. The fact that you got through 5-6 years of medical school (having had good feedback and done well in your finals, by the sounds of things) is a good indicator of your ability! I'm sorry you had such a bad experience on your last placement... But sometimes people just don't click for whatever reason, and the fact that the F1 as making you feel that way was probably more a reflection of him, or his working environment, than you. Try and put this behind you and focus on the fact that you have passed your exams and all the reasons why you will be a good doctor!
It's a shame you didn't get the jobs you wanted, however you might actually find that you enjoy them a lot more than you think. My favourite job (and one I got the most out of) turned out to be the one I was looking forward to the least. As for declining a job this year and replying, I have never heard of anyone doing that, but maybe someone else will be able to advise on whether this is possible.
Ultimately, whether they admit it or not, the vast majority of people are worried about starting FY1. It's a huge transition, and regardless of how much time you spent on the wards as a student, you will not appreciate what it's like being a junior doctor until you start working. You will be tired, and stressed, and you will make mistakes, however there will always be someone more senior around to ask for help, and you should never be scared of doing so. People will expect you to be unsure of what to do, and will be more than happy to help, especially in the first few months. Make the most of your shadowing weak - learn what the paperwork looks like, where to find local guidelines, where equipment is kept, how to order bloods, scans etc., and this will make you first day of work slightly less stressful. Make friends with the nurses and make sure they know it's your first job, and this will also make life a bit easier.
It's normal to be worried and I am sure you will be absolutely fine
If you have any questions or anything specific you would like advice on, feel free to message me and I will do my best to answer.