Consider what you want to do at university and look at the subject requirements. I'd recommend keeping Chemistry as it will keep your options pretty open for studying almost any science subject at uni.
As someone with this combo (plus Further Maths), Chemistry can be a bit of a break from constantly staring at numbers.
If you're worried about falling behind, I'd recommend going through and compiling your notes at the end of every week, if not every lesson. Definitely read through the relevant sections in your textbook and note down anything you still don't understand so you can ask your teachers. Don't do what I did and pretend to understand everything right until the end of the year!
As far as Edexcel Chemistry is concerned, I'm afraid the revision guides are incomplete. If you're desperate for a revision guide, your exam syllabus is pretty comprehensive (and free), but the only downside is that it is more of a checklist. On the other hand, it does contain everything you need to know by definition, so you can be sure you won't miss anything.