I started driving in March last year, I had a couple of lessons and was doing quite well, and I managed to practise in my dad's car once. Lockdown happened, and I obviously couldn't have lessons for a while, nor could I practise in my dad's car as he lives a few hours away and I didn't see him for a few months at the very start of lockdown. When I did eventually start practising in his car again in about June/July, I was very rusty and needed reminding how to do the very basic things that I'd been fine with before. We started off by driving around the car park at a retail park (it was usually early evening and retail was still closed so there was no one around most of the time), and it took a few sessions there before I felt confident enough to go on the roads again. Moving from my instructor in her car to my dad in his car was quite difficult for someone who hadn't driven for a while, but I got much better in time and was driving for several hours at a time along some tricky routes by the end of the summer (he lives near a lot of hills and country roads which were pretty challenging for a new driver). I had my test booked for early April which was obviously cancelled due to COVID, but I'm fairly confident that if we hadn't had the 3rd lockdown and my test went ahead, I'd have probably passed. In the last fortnight, I've had my first couple of lessons back with my instructor since December and she says I'm doing fine, and will be test-ready again in a few weeks.
Long story short, not driving for a while - especially as a new driver with a new instructor/car - will mean that you'll forget things and struggle to get back into it at first. Don't worry about it and don't give up, learning to drive during the pandemic with all the changes regarding lessons has been a pain. Pre-pandemic, I figured I'd have passed in much less than a year, but now it's nearly 14 months since I started learning and I haven't. COVID's screwed over a lot of learner drivers this year, don't beat yourself up if you're struggling to get back into it. It'll come back to you, and if not, maybe try a different instructor or go back to your old one.