Don't feel bad about the threads/responses you are seeing on VCE notes. There are definitely a fair few people doing further maths that really shouldn't be (i.e. their abilities exceed the work) but there is a very much disproportionate representation on these forums, and those who are exceedingly good at maths simply make the 43-50 range very competitive. There is an overwhelming number of students who are of very limited mathematical ability doing further and many also give up, as long as you keep working at it, construct yourself a decent bound reference and make the most of your time before the exam you will easily surpass the average further maths student no matter your base ability.
Although I understand how it can be to do work/revise for a subject you only understand a limited amount of the subject material for (I essentially got lazy sem 2 and stopped doing methods work altogether now I'll have to attempt to revise the entire course in a day). If you are really struggling to understand some concepts just don't bother with them, ensure though that you know all of the coursework you are capable of understanding very well and you'll do fine.