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.