(Firstly, let's ignore the school rank factor. Last place at my school aced 3U and struggled with 4U, but got 82 anyway.)
This is just the reality of taking on such a demanding course. Concepts are not meant to be obvious and only a few people will understand everything on the fly. You're gonna get stuck all the time with 4U concepts and questions and then just not feel like doing anything.
That's the typical part. What matters is how you handle it.
Let's start by considering just how many past papers have you attempted thus far in preparation for each assessment task to date. For something like internals, maybe not much is really needed (2 is a good number). The exception so far would be the half-yearly exams, where 4 or 6 would've been a good number. You should ask yourself if you genuinely gave it a shot at these papers, not whether or not you've managed to complete the entire thing. How long did you actually spend trying to do a question? The later the question appears in the paper, the more time should be devoted to it. (Difficulty in maths paper questions is usually in ascending order.)
And now that you're at the trials, you decide for yourself how many you should do (because obviously you have to study for all the other subjects as well). I will not give you a number because that's just adding unnecessary pressure.
Now, consider how you approach past papers. Are you commencing open book BEFORE closed book? Do you push yourself to a point where you're genuinely stuck before you give up? And when you're stuck, are you brave enough to actually check AND copy the answers? An important part of studying for maths is to actually write down what the answers are and not just read them. Short reason why: Writing forces the brain to retain methods and processes more.
And the important thing is to actually force yourself into it. Nobody likes to do it, but it's absolutely necessary. Employ strategies such as studying at libraries and limited use of technology to force yourself to do questions. Print out past paper questions and answers if necessary - there are tons of past papers at sites such as THSC which you should be well aware of. The important thing is to force yourself, because if you can't do that then nothing else follows.
(There's a small section on exam tips at my lecture. Might help a little.)