Hi!
First, create a schedule or to-do list! Having a bit of organisation makes you feel like you have your study in order, and this way, you'll feel like you have a bit of your life in order hahaha. Since you seem a little short on time, just pick out things you know you're not as confident at. E.g. maths, focus more on topics/questions you're struggling with (still practice everything else!) going through past papers and picking out questions is a good way to go. It's the same with other subjects (can't speak for bio or eco), but pick out things you don't feel as confident in, and focus on them a bit more. If worse comes to worse, cram lol it's not the best option, but at least you have that peace of mind knowing that you've done something.
Have you looked at any of your school's past papers? Identifying trends can help you predict how they may ask questions and give you a good grasp of what they may ask. :-)
Stay chill, doing crap in half yearlies doesn't guarantee you'll do badly the rest of the year - I flunked all my half yearlies (like, real bad) and came out alright lol. Don't focus on results, just focus on preparation, because you'll feel so much better going in knowing you've tried your absolute best. :-) All the best!