I did AQA Chemistry A Level and the advice I would give is to do question after question after question. As soon as you have completed a topic, do the questions to make sure you have understood it. You can normally find the mark schemes online or ask your teacher to mark it for you.
Also, use flashcards and post-it notes, they're amazing for writing down definitions that you need to know by heart to obtain marks.
I am a visual learner so I also often watched videos like SnapRevise to help me absorb information after I had finished reading about a topic in my textbook. Mind maps are also amazing to use.
Since you are Year 13, I suggest you start making notes that can also be used as revision material, this will especially help as you won't need to worry about it later on because you will have everything you need on hand.
As for not being able to focus, I totally get that, I was the same. You do need to discipline yourself though otherwise you will regret it when you have to sit your exam. Don't have any distractions around you whilst you're working, get proper sleep and give yourself breaks so you don't lose motivation.
Hope this helps