Sorry your results were not what you are hoping for.
Very very few people go from a D or even C at NAT5 to an A at Higher, statisically many with those grades in NAT5 wont even pass higher. Your school knows this, they know higher is a big step up, and they are trying to support you to get better highers in the areas you have shown strengths in instead.
For entry to medicine at uni you will need AAAAA at Higher in S5, spaces are extremely limited and you will be competing with straight A students for those spaces. You need to be honest with yourself if this is realistic for you. The jump in science and maths from NAT5 to higher is big, especially when you havent grasped the NAT5 basics, and taking two sciences from C and D to higher AA is very unlikely.
Your best bet is to get an appointment with your careers advisor and Universities to see if there are alternative routes or careers in a similar area you might be interested in.