Imposter syndrome is very real isn't it?
I can assure you that you are absolutely not alone. Term 1 of year 1 is often the hardest as it is such a shift in pace and method of learning. To be fair is quite hard on the teaching staff too. They know that everybody is starting in different basis of knowledge, they are having to try and bring everybody along at the same time with out losing anybody along the way.
You could think of the formative exam as being as much a test of how well they are doing their job and of how well the cohort is starting to have broadly similar understanding as it is to help you gain an understanding of how you are doing.
Some will do well, some ok, and some will do less well. That is the nature of exams. Afterwards it is an excellent opportunity to reflect on how you have done, and importantly what you have done to learn, and how you might wish to enhance or adjust / mix up your study skills approach moving forwards. Anki works, but not for everyone, and if you skip a bit or miss a day or two it can feel like an impossible hill to climb.
Speak to others on your course and no doubt you will find others are feeling the same. Talk to them about their study approaches. If you have med parents on your course,ask them, or any friendly (or not) 2/3 year. Or post in the current med students and Doctors forum on TSR and ask others for hints and tips. None of this means that you can't do it. You can. You just have to remind yourself you can.
Good luck with the exam. And spare a thought for all the other students who are having their Christmas holidays ruined by having to study for summative exams in the Jan window !