You are predicted to get good A Level grades so I wouldn't consider changing them unless you absolutely have to. They're useful and you will be able to apply to many courses with them should you decide to change careers. Of course, if you end up specifically needing English or something then do change them but if you aren't sure yet then it's probably best to just stick with them as the sciences are very applicable to many fields, and you don't want to end up doing another year at college when you didn't need to.
Ultimately, being a vet can be rubbish. But it can also be very rewarding. Not every vet clinic will make you work these long hours and the pay isn't always terrible. Having said that, it is quite common in our industry to experience these things: in about 2019ish, an article was published that found more than 20% of vets were seriously considering leaving the profession soon.
My favourite thing about veterinary medicine for me is the diversity in careers. You can be a general small animal vet and make a difference to many families and their pet's lives. You can be a farm vet and actually get to work outside in all weathers and a variety of settings. You can be a veterinary specialist where you do extra training in one area and then work in a hospital on complicated cases - the pay for this kind of work is usually much better. If you still want something completely different you can work in a laboratory research facility where animals are used, supervising their welfare. You can even work more office-based jobs for the government advising for public health (the work-life balance would probably be a lot better for a role like this).
So is it 'not worth' becoming a vet? Well that depends on who you are. If you genuinely like the job role - performing surgeries, seeing patients, decision making etc. then there is no reason that you can't become a vet and apply to jobs where you can set strict boundaries on your work-life balance.
I'll also add that almost every vet I have ever met (which are mostly women) have children. The pay isn't great for the amount of work we do (and how difficult it is) but is definitely livable and you can always get extra qualifications to help increase this. Unfortunately however, if pay is one of the main things you care about then this is not the job for you. Being a vet is rewarding in ways other than just compensation and the job is quite emotionally draining - there are much easier jobs that pay more to be completely honest.
Putting aside the hours and pay - did you like the clinic environment? Did you enjoy watching the surgeries, working with patients etc? If so, maybe it is worth getting some more placements done at other clinics to see what it is like. You may have just gone to a really poorly managed clinic. We are expected to see any end-of-day emergencies that come in, however 20 hours of overtime certainly is not the norm.
You don't have to figure out what is best for you by the end of Year 13. Almost no one knows. Some people don't even figure out their career until they're in their 30s. I actually took two gap years before I got into vet school. Take your time before rushing into vet, law, dentistry etc. If you apply without being sure, you might feel pressured to take your place and then end up doing something which is completely not right for you. One thing for sure is that veterinary isn't for everyone. You have already seen that the job can be tough and I wouldn't want you to rush into that.
If there are any more specific concerns or worries you have feel free to ask me any questions. I hope you figure it out.