Fear is somewhat the perception that a situation is of "life altering importance" in combination with the perception of being unable to handle it.
Also worth nothing this is just the perception of it, it neither means that the situation has to be life altering, the importance can be "objective" (like being eaten by a lion is something that a bystander is able to readily comprehend) or it could be subjective like making a step towards someone you like may not be seen like much for a bystander, but it could feel like the most important decision in your entire life to you. Likewise the situation might be actually severely dangerous but you might not perceive it as such.
Same goes for your ability to handle it, you might be unable to handle it and be fearless nonetheless or you might have all the tools to deal with it and more and you still might feel ill-equipped.
Also what do you mean by "unjustified assumptions"? That sounds like the problem is just being wrong or making assumptions. But the problem in many situations is that you're confronted with an uncertainty, not only about which outcomes will actually happen, but even what the possible outcomes will be and how they will "feel", yet in terms of fear you expect them to be bad.
And that assumption can be both justified or unjustified. And whether you stress about it is not related to that question. Like let me assure you it does not fill you with serenity to face a test if you haven't actively practiced beforehand. Removing the uncertainty about the outcome of the test kinda misses the point that what you're anxiousness about might not be the test itself but what comes after the test and that this is outside of your control and your knowledge but it's likely not positive. (Also not learning for a test isn't assuring instant failure either, you're learning all the time so just because you're not actively doing it doesn't mean you're not still doing it).
Likewise what's bothering is not whether the assumption is true of false but that you don't know how to handle the situation. Like often enough the exact worst case scenario actually happens and it's not a big deal. You might fail your first attempt? Who gives a shit. It's likely to end up being one of hundreds of failures in thousands of attempts, but if it's the first it's something that you don't fell equipped to handle and so you're running "what if"-scenarios in your head without knowing their outcome, probability or significance and increase the feeling of inadequacy.
Now most of them end up being wrong or different from your expectation, but are they therefor unjustified? Like I've recently read the advice that you should actually fully sketch out the worst nightmare scenario of yours and walk through it in your mind, that way you might build up the confidence by being able to prepare for something, rather than being overwhelmed by an abstract feeling of being powerless in the face of a threat.