Grammar is so important for the gcse so i would highly recommend you just sit down and take the time to learn all the verb conjugations for ar er and ir
this is stupid but it gets stuck in your head (for preterite conjugations). Once you've learned preterite tense, the other tenses mostly follow a nicer pattern.
I would also recommend that you take the time to learn verb conjugations for some of the more relevant irregular verbs.
Mostly, don't stress you still have over a year and you will likely find it far less difficult than you're anticipating,
good luck with revision