What is it that you found "overwhelming" in 3rd year? Your later clinical years are very different from the first few years of medical school, and it's a new challenge to try and get used to them - I know people who flew through pre-clinical and really struggled at clinical school, and vice versa. You mention the endless grind of exams/hoop jumping, and it sounds like you were rather intimidated by clinical work, but you still find the subject interesting. If you took away the exams, do you think you would enjoy actually studying medicine? If so, then press on and just accept the exams as an unpleasant but necessary hurdle. If you don't actually like medicine any more and wonder if you want to be a doctor, then it might be time to look for a way out.
As for hating FY1/2, I think it's a very mixed bag. I'm sure there are some people who hate it the whole time, but I expect there are far more people like me, who hated the long hours, the lack of appreciation and the crapness of NHS administration/understaffing, but actually deep down like seeing patients and looking after them. It's easy to feel totally overworked and stressed, but at the same time realise that you're really little more than a glorified secretary, especially as an FY1 - that sucks. I managed to deal with it by learning to take pride in what I did - even if it was "only" secretarial work, it is really valuable to the team to have an up-to-date list, to know what investigations have been requested for which patients and what the results are when they arrive and so on. Even though I wasn't saving lives all the time, and spent a lot of time writing TTOs, I liked to think I was keeping things running smoothly so my seniors could get on with the clever stuff. As an FY2, I got more responsibility, working in A&E and doing SHO-level medicine on-calls, and although it was knackering, it got more satisfying. Now I've done a year in a non-training post in ICU and I'm a CT1 in anaesthetics, and I love my job. Sure, the hours can be crappy, and it can get stressful, but I very rarely go home thinking "I wish I hadn't gone to work today."