The closest to being colloquial is one of the following:
jumping to conclusion or
if you are just being careless in not considering all the evidence. On the other hand, you are
stacking the deck, or
if you know there is relevant evidence for a different conclusion, but you are purposefully choosing to ignore it.
Let's be clear what the actual fallacy is.
It is not that the data in question is irrelevant. Indeed, the data on its own does tend to support the claim that the class is difficult. After all, other things being equal, the more difficult the class, the more likely it is that you'll have to still be stressfully studying two days before the exam.
The problem, however, is that other things are not equal. You are not looking at all the relevant evidence, notably the fact that you haven't studied at all until the finals week.
Jumping to conclusion/suppressing the evidence
If you are doing this unknowingly, then you are jumping to conclusions (see here), i.e. making decisions or forming opinions before one has all the pertinent facts (source). Another way to say it is that you are making a hasty judgment (source).
On the other hand, if you are doing this knowingly, then you are suppressed evidence, providing a one-sided argument. This is also known as card stacking, stacking the deck, ignoring the counterevidence, and slanting (see here).
Philosophy professor Peter Suber explains it like this:
The one-sidedness fallacy does not make an argument invalid. It may not even make the argument unsound. The fallacy consists in persuading readers, and perhaps ourselves, that we have said enough to tilt the scale of evidence and therefore enough to justify a judgment. If we have been one-sided, though, then we haven't yet said enough to justify a judgment. The arguments on the other side may be stronger than our own. We won't know until we examine them.
So the one-sidedness fallacy doesn't mean that your premises are false or irrelevant, only that they are incomplete. You may have appealed only to relevant considerations, but you haven't yet appealed to all relevant considerations.
Affirming the consequent
The following is probably true:
if the class is (very, very) difficult, then I have to stressfully study right up to the exam.
In fact, let's grant the truth of it. The problem is that your argument then proceeds as follows:
I have to stressfully study right up to the exam.
Therefore; the class is (very, very) difficult.
That argument is invalid. It has the following form:
If A, then B.
(invalid; the fallacy of affirming the consequent)
What would be valid is either
If A, then B.
(valid; modus ponens)
If A, then B.
(valid; modus tollens).
Given the conditional if A, then B, in logic, A is called the antecedent, and B is called the consequent.1 What you've done is affirmed that the consequent is true, and from that concluded that the antecedent is true. And that's invalid. To conclude that A is true, what you need to be true is not if A, then B, but rather its converse, if B, then A.
1In linguistics, A is called the protasis, and B the apodosis.
One can also say that you have committed the fallacy of confusion of necessity and sufficiency. Given the conditional if A, then B, we say that A is a sufficient condition for B (which makes sense, given modus ponens), and that B is a necessary condition for A (which makes sense, given modus tollens). But in affirming the consequent, the argument proceeds as if B were a sufficient condition for A. And the truth of if A, then B does nothing to establish that. Granted, for all you know, it might be the case that B is also a sufficient condition for A (in which case it will be a necessary and sufficient condition), but also, for all you know, it might not be. You just don't know, if all you know is that if A, then B.
This fallacy arises when there are multiple sufficient conditions for B to be true. For example, suppose B is the room is dark. All of the following are true (provided it's nighttime, and there are no other sources of light, and the electricity is on):
If the lamp is broken, then the room is dark.
If the lamp is in good working order but unplugged, then the room is dark.
If the lamp is in good working order and plugged in but is switched off, then the room is dark.
and countless others (e.g. If the lamp is in fact on and working but is covered with an opaque box, then the room is dark). Now suppose that all you know is that the room is dark. Then you can't conclude that the lamp is broken; maybe it's unplugged, or switched off, or covered with a box, etc. You need additional information to determine the true reason why the room is dark.
And then, as we discussed above, if you are not aware of this additional evidence but still choose to immediately conclude that the lamp is broken, you are jumping to conclusions. And if you are aware of additional evidence pointing to alternative reasons why the room may be dark, but choose not to consider it, then you are suppressing evidence.