If you properly break it down into paragraphs rather than a wall of text you're more likely to get a response
1) You do not need to go to Vet School or university to study to be a veterinary nurse, you can study Veterinary Nursing in a number of colleges across the country (ie; Diploma in Veterinary Nursing - City & Guilds Qualification).
2) Get plenty of work experience before you commit to a course. Veterinary nursing is not as glamorous as many people think it is, and by and large it is poorly paid.
3) Seriously consider the Nursing Diploma/City & Guilds route as opposed to studying Veterinary Nursing at University given your prior issues. I suspect the more vocational course and learning environment would suit someone like yourself better than the more formal/academic environment of university.
4) You can go back to college to study for further qualifications, whether that is to study A-levels if you are currently doing extra GCSEs now, or for the Diploma in Veterinary Nursing post A-levels, there isn't a 2 year lifelong cap. However you would probably have to pay course costs past a certain point, depending upon your age, the level of qualifications you have and your local authority/government.
5) You will almost certainly have to achieve decent passes in your GCSEs before progressing to A-levels. The chances of getting a decent A-level pass without a decent GCSE pass in hand is unlikely. Some colleges offer accelerated single year A-level courses although this is not something offered everywhere, it can be expensive and you need decent GCSE passes (or plenty of tutoring/support) to make it work.
6) The Nursing Degree route versus the Nursing Diploma route has been discussed at length before (search the forum). Both have advantages and disadvantages. The nursing degree route does not guarantee better job prospects despite what some would have you believe and a lot of your success depends on your prior experience, work ethic and how you apply yourself.
7) What you are thinking about is achievable but you have a lot of work ahead of you (ie; academically and work experience) and it may take you a couple of years longer than you think to be in the position to apply. That in itself is neither a good or a bad thing, a couple of years is nothing over the course of your career and you end up entering the jobs market with more life experience (hopefully!).