I completely understand, I'm in year 11 now but I had to do my core science in year 9 and didn't bother revising and I got a B over all which I guess isn't bad, but I knew I could do better and it's honestly the worst feeling mate.
Just do it, if you think about it, it's only for a short while that you'd have to revise and if you get good results it would be worth it, don't be that person (me) who knew they could have done better but didn't bother trying!