If I resent your doing A and you didn't do A, then my resentment was perhaps justified (if I was justified in thinking you did A) but it was nonetheless misplaced. On the other hand, if I am crossing the road and I notice a car speeding towards me, and I fear it will run me over, but then the driver brakes and stops just barely in time, my fear was entirely appropriate and not at all misplaced.
The proper object of resentment, thus, is an event (or action) taken as actual (and wrongful), and when that action doesn't occur, the resentment is misplaced. But the proper object of fear is an event merely taken to be a serious chance. What kind of chance? An objective chance or a merely epistemic probability?
I will argue that it's an epistemic probability. Suppose that I fear that my investments will fail. I get into a time machine, travel to the future, and notice that my investments won't in fact fail. I go back in time and it would be appropriate for my fear to go away. Nonetheless, there is an objective chance of the investments failing: the chancy processes that make investments go up and down continue to run despite my knowledge. But there is no longer a serious epistemic probability. So it looks like epistemic probability is what is relevant. Moreover, I think it can be appropriate to have fears about things that are in fact necessarily false. For instance, if I have an answering a multiple choice exam in calculus, and I the question asks whether the definite integral of some function over some range range is 2, 3, 5, or π/2. I think it's probably 5, but there is something in my calculation that I am not confident of, and I realize that if I got that wrong the answer is π/2. My fear that the definite integral might be equal to π/2 is in fact appropriate, even if it is necessarily true that the answer is 5.
This makes fear very different from resentment: fear is made appropriate by epistemic probabilities--either the actual ones or the ones my evidence justifies (which one?), while resentment is made appropriate by what people have actually done.
I wonder if this focus on the epistemic dimension isn't partly responsible for the notorious way that fears resist rational thought. No matter how much I reflect on the very good statistics for indoor wall climbing injuries (the chance of injury during a session is about the same as that while driving 26 miles) and what I know about the stringency of Baylor's training of my belayer, when I look down from 50 feet up, I feel fear. This fear is misplaced: my epistemic probability for a fall is tiny (and justifiedly so given the evidence). Why? Because it looks dangerous. Now, in the absence of defeaters, appearances yield epistemic probabilities. Moreover, many times even though a defeater to an appearance of an impending bad is sufficient to defeat belief, a sufficient epistemic probability will remain (after all, we may be wrong about the defeater), and it could take quite a bit of time to evaluate whether the defeater is complete or only partial. Given that physical danger may require a quick response, and the examination of defeaters takes time, it makes sense for us to be wired in such a way that appearances have a strong tendency to directly drive fear. So in cases like my climbing case, while the fear is misplaced, inappropriate and unjustified, it is nonetheless understandable (unlike my pathological fear of dogs!).
(Well, when I reflect on the fact that an indoor climbing session has equal injury probabilities to a 26 mile drive, this actually makes me feel a bit afraid. For I do think driving (or being driven) by an average driver is genuinely dangerous. And so perhaps my fear is justified, just as I would be justified to be afraid of a 26 mile drive (even if in fact I don't always feel afraid). If so, then change the example, say to standing on a five inch thick glass floor above a precipice.)