Hello.
Firstly, is the grade actually "terrible" (I hate saying this word) as in you have completely and utterly failed the mock and got 0/100 on a paper for example? Or is your idea of a bad grade a C or B grade (60%+ usually on A level papers from memory)? Because at A levels an E is a pass, so getting a C or a B at this stage is good as it is well over a pass and means that you are on the right track to possibly/potentially getting a B or A in the exams if you keep pushing on.
Secondly, I understand how you are feeling. I'm not going to go into detail so just believe me when I say I understand where you are coming from - really I do. When you are severely anxious and significantly 'down', a lot of the time your mind isn't in the place it needs to be and is somewhere else, so this could be the problem. May I ask (I know this is a public form but it might give you some clarity) is there something, or an underlying reason (or reasons) why you feel this way? Because what you are writing about here (I am no mental health professional) sounds like criteria for severe mental anxiety & depression to me. I also understand that May and June are final exams season, so this may be making it worse - but maybe talk to a GP about this, as for some people if you go down the CBT (Cognitive Behavioural Therapy) it can help them, or at least give them methods and ways to deal with these persisting and life affecting negative feelings.
My PM is always open my friend. Remember no matter what happens in the end you are not stupid, you are not inferior to anyone else and you are not alone.