I'd say put your foot down and stick it out with the A levels... I mean, what will you do if you're not at college? I would assume that having all that time to think about things would make it worse.
On a practical side, A levels are a lot harder than GCSEs so I'd make sure you can cope - if she has a point, don't be too scared to admit it. Maybe go for the first few months and see how it goes? If you take your AS levels and they go badly then there're always retakes, very few people DON'T do some so it's not the end of the world. Another idea would be to wait for your GCSE results and see how you did in that - that'll be an indication of your coping strategies etc.
Meanwhile, make sure you've got some decent medication from your doctor if you feel you need it, don't be put off by them and ask for a second opinion if they're reluctant to prescribe something. Maybe take someone with you for moral support on this one?
Ultimately, it's your decision and you'll have to have a proper talk with your mum about it. Make sure you know what you want and go for it, and let her know you need her support in your decision.
Hope you sort things out,
skevvybritt x