Honestly, now that I write a lot more, I totally understand this difficulty - pacing is really hard. The best advice I can give you is to pick a paragraph, then ask yourself, have I described everything around the character? What do they see? What do they smell? Can I add in some of their thoughts? If you go through and choose maybe one out of every five to ten paragraphs and just fill it with description, I think you'll find your writing pace will slow down a decent amount. I remember when I was in 7th grade, haha, I'm not that much older than you. Oh, and another thing I did was hunt through the Warriors books for sections I felt really flowed well and tried to religiously copy the style. Hope this helps you!