Are you doing A-level maths? There's quite a lot of overlap with the mechanics section. You will get more practice with it and you'll be better than you are now.
A lot of mechanics problems in A-level physics just involve being able to apply and rearrange equations of motion for constant acceleration ("suvat" equations), or correctly applying conversation of energy/momentum (such as friction slowing something down, or inelastic collisions), or just getting your head around moments. There's not much that's beyond the GCSE.
If you've only just started year 12, and you've just had the longest holiday you've ever had, it's pretty natural that you'd be rusty on it. Just keep practising questions, starting off easy but getting more difficult.