Firstly, its quite ok not to know what you want to do at year 12. Many people 20 years on still don't know what they want to do, either in life or in study! The best thing here is you know you are undecided and you can take your time making decisions. If you explore all of your options and then visualise yourself doing something then by the time you do choose you will probably have made the right choice for you. That matters. Do you love the subjects you have chosen? Was that your decision to study those subjects? Do you have any health issues that would hold you back?
Where are your basic interests - sciences, maths, humanities. Know what makes you tick and feel alive? If you are good at maths & sciences they will open virtually every occupational door going for you (even not so good grades) Your next year will be the tough one as the A level work cranks more to applying your fact based knowledge to new scenarios and theoretical circumstances. You have to love the subjects you are taking (or at least have a vested interest) Talk any decisions through with your tutors. That is so important to see what many other people think and know. Then trust your own judgement and make your own decisions. You need to take the responsibility for your own choices but try and keep your parents onside as you may need them at some stage. If it goes right great, and if it goes wrong - not the end of the world go to plan B or plan C etc
Why do you want to do year 12 again? What do you hope to achieve by doing this? What isn't right?
Sometimes Medicine or Vet school can be quite subject specific at the admission faculty, so if that is what you want to do get some advice and see if your grades are ok? A year out would have to be used productively for life experience preferably with animals or people, so it is still learning not festering and wasting your time. Use the medicine forum on this site if that is what you are thinking.
Anything else - It is vital you work as hard as you can to achieve maximum possible grades. At this age you should have no family commitments and can selfishly devote all of your time to your own needs. The mental pain is temporary and you can take your foot off the loud pedal a little when you get to Uni when you have arrived at the place you want to be.
Start thinking about the type of job you see yourself doing and working at in 5 to 6 years time down the line.
Outdoors/Indoors?
Predictable hours, or shift work (are you a night owl/lark?)
Work as part of a team or solo?
Do you like people? (or prefer animals)
Excitement and adrenaline/Extroversion?
Predictable work/Introversion?
Love Money / Love material goods/ Love human values?
Sports or sports psychology?
Rules based - armed forces (Navy, RAF, Army) border force, police?
Money services - people or IT based, Banking, Investment, Insurance, Housing
Engineering - construction planning & implementation
Cultural - Tourism, arts, history services, dance, theatre
Public Services - NHS, Dentistry, Local Council, Education, Housing, Transport
Teaching, Research,
Farming, Forestry work, Land based work
Utilities - Gas, Electric, Water
Mechanical - Electrical, Vehicular, Scientific
Reporter/Journalism - Local, National
Manufacturing - Production line, small business,
Food Production
Languages - translator, civil service, embassy work etc
Retail Management (shop floor work badly paid) / Hotel work
So then take a few courses that jump out at you and research what the Uni's usually want as grades and what they offer. Get out to the Uni's and talk to some of the students. Is it good to study here? What is campus life like? Do I get good vibes from this Uni? How many post grad students get employment? What are the fees? What is accommodation like? What are student services like?
Then research apprenticeships as an option - BT, Gas, Engine manufacturers, Aerospace, etc and ask yourself if you really need a degree.
Year 12 A levels are usually a good predictor of the end of year 13 results. Why give up or start again when you could double down your efforts and ace it at the end of another year.
Best of luck but don't agonise - just get on and do it. Good luck (& never waste a moment in this life - time can never be relived)