I think you feel discouraged when you are doing bad in a subject, making you dislike the subject and getting bad marks everytime.
You just have to work harder and have more motivation by setting long and short term goals (for example 95+ ATAR or that uni course you're dreaming about doing.)...
If you think your worst subject is maths and you're struggling in it then drop it. There is no point doing it if you think it won't count into your 10 units.
Either way, if you're worst mark is mathematics, it won't count into your 10 units. If you think you're not good in Mathematics then I'd say drop it and focus on your other subjects, because there will be no point doing it for the extra 2 units.
It isn't a must, but it's recommended that you do it so that you find it much easier to understand your course. Also, there a bridging courses which you can do in uni later on to catch up with chemistry if you haven't done it in year 12.