It seems you are doing plenty of revision already, and once you find this helpful then you are already on a great start.
I'd suggest doing 2-3 hours after school as well, as we're in March and now is the time to push it as exams are looming closer for all of us.
As for not losing motivation, do subjects that you are struggling on and try to make learning fun. Either use flashcards to play a game of quizzing yourself, ask a family member/friend to stimulate your learning instead of doing it yourself, or use sites like Quizlet. Then, when you've grasped a topic in one of these hard subjects, I guarantee you'll feel more motivated to do another topic and it will start becoming easier. It's like doing a hard round of exercise and feeling the burn a day after - yes it hurts, but suddenly you feel more motivated because the exercise you're doing is working and you'll feel and see the reward of that soon.