A single grade 8 piece of work probably doesn't show your overall performance, and so your English teacher is likely to understand that you may not be at the level for a predicted grade 8. Its not very good to be too perfectionist, so I would recommend looking at the feedback and understand how you could improve to do better when you're in the real exams. Grades normally increase from mocks, but not without any work put in! Worrying too much about a single number really isn't good for you (I've learned from experience lol) and stay positive! A 7 is still a really good grade and have hope (my english lang and lit gcse grades were 2 grades higher than my mock grades, so don't feel too down about it).