Hey! I'm a graduate medic myself and used to feel that all the time. Still do! Have a science degree, but a bunch in my course are from non-traditional backgrounds, such as business. We're all doing alright really.
Had very shoddy a-levels because of struggling with my mental health, built my confidence and resilience a lot up at university during my first degree. I personally feel looking back, at 18 I would have never been that emotionally mature to pick up a career of long-term commitment which is medicine. Given the amount of content involved, things do get stressful? It is just a lot of hard work and pure determination, you're not going to be spoon-fed and must go back to read up other sources to supplement your knowledge. I know this is where I want to be in life, so I always make the effort to ask questions, pick up any opportunity to improve my clinical skills on placement, provide revision lectures to the younger years as it reinforces my previous knowledge too etc... it's not about how smart you are at all but rather how much initiative you put in. I don't consider myself as particularly bright or intelligent by any means, but I want to do whatever it takes to ensure I can do the best I can for my future patients later down the line.
Think if you know this is for you, start by getting work experience and really finding out why medicine? Speak to the staff on wards, understand the ins and outs of the medical career (the best way you can do this is by actually being there, in the moment?). Warwick GEM also pay a lot of attention to your UCAT I believe, so definitely investigate that as well. You seem you'll be great, good luck!