If medicine's really your dream, then I say stick with it. There are plenty of avenues open to you in order to get to where you want to be. You could, and would perhaps be better off resitting your AS grades to try and bump them up, as A-levels are the most straight forward way of getting into medicine. If that doesn't go to plan, you could pick different subjects that you're better suited to (e.g. Humanities) and then apply for Foundation entry. There's always the option of graduate entry medicine, in which case you'd need to pursue a degree first. (Where you apply would also factor in, as some universities would still look at and question your poor AS) There's also the option of waiting a few years and pursuing something else, to perhaps look at Access to Medicine courses later on either.
You can get there eventually, don't give up!