Hey there,
Just like all mods are saying, your half yearly marks don't matter as much. To be honest, this is something I've been trying to get in terms with because I basically break down after all my exams from a mix of pre and post-exam stress.
My math coordinator basically reminded me that there is still 60% of your assessments that you haven't completed yet. Even then, trials don't contribute as much. I remember Jake, a HSC lecturer, saying that he didn't even do a whole page in his 3U math paper in his trials and he still got a 99+ ATAR.
Think about it this way: even though your marks look bleak right now, the difficulty of your paper will differ to the difficulty of my exams or the difficulty in other schools. You teacher could've made your exam really easy and help everyone get 90%. Or they could've made it hard so you're used to the difficulty when you get to the HSC. The things that matter are your ranks and your HSC. As hard as it is, it's better to get roasted now than during the HSC because your HSC is what counts.
And another point, ATAR calculators are never reliable. They're only using the previous years' results and compare it to your marks. There are too many variables that make this an unfair test; the 2016 paper might be harder than the 2018 paper, there may be more people doing better in the previous years than the 2018 cohort- there are so many differences to the point that putting your marks in an ATAR calculator is purely useless. Plus, it's really bad for your self-esteem. I had to force myself to stop putting my marks in the ATAR calculator because it's never an indication of your actual ATAR.
And remember, a handful of us are on the same boat as you!