I'm sorry to hear you're disappointed with your results! As someone who did VCE 10 years ago (but still tutors VCE students) I can tell you that your ATAR isn't the end of the world. Getting a 77 is pretty good and if you're determined to get into Arts at Melbourne or Monash, it's quite easy to study at a university that has a lower ATAR requirement and then transfer across after 6 months or a year. I hope you do get into a course you like and do well!
In terms of your actual score, you wouldn't have been disadvantaged from only doing humanities or from your school - the student I tutored for English only did humanities (English, Media, History (Revs), Drama and Health & Human Development) and she achieved an ATAR score of 93.5 and became Dux of her school. Although she went to a "Private" school, it was one that was out in the suburbs, was not really competitive, and many of the students only achieved study scores in the mid 20s-30s. My student was the only one to get in the 40s for English, everyone else was in the lower 30s and 20s.
I find the biggest problem that students face, particularly those in less competitive and disadvantaged schools is that they don't realise just how much competition they're actually facing. As a result, they don't put in enough time & strategy into their studies to reach their desired scores - Students need to know all their subjects inside out and write as many essays and practice exams as possible if they want 90+ scores. They also need to remember that VCE isn't just about studying, it's about how much you've worked relative to other students in your year level across the state - there will always be those students who are naturally gifted or who study for a crazy number of hours per day...so students should consider how they would do compared to those people in an exam and work harder if they desire similar high scores. Unfortunately, in disadvantaged schools, teachers don't often emphasise this for fear of disavantaged students feeling pressure and dropping out of school.
As a student who attended a selective school in the city myself, I was constantly reminded of this by my teachers and tutors to motivate me to study - And I think that's one reason why "Elite" schools in the city, and other populated areas, do so well. They are reminded about how much competition there is, but also given more support and materials to help them to do better. Some students scoff when I say this to them, but when I show them the scale of how many students are in Victoria, and how many attend selective schools or accelerated classes, or have had private tutors since they were young...they start to get the idea about how much harder they might have to work. It's not to scare students, but to give them a realistic idea of their competition and to help them with setting their study goals leading up to their exams.
Anyway, I think it can be hard to feel like you haven't achieved what you wanted to, but you still have a lot of time and potential. When you search for a job in the future (which I assume is the end goal of your studies), people care more about your university scores, what course you studied and your practical experience more than your ATAR score. My advice would be to pick a course closest to what you want to do, maintain a decent average and transfer into your dream course. Good luck