There seems to be nothing incoherent in the idea that there might be certain people who are much more reliable about the future than the rest of us, though neither we nor they can account for the source of this extraordinary talent. I don't think it is likely that there are any such people about; but if there are this might have an entirely natural explanation (even if we can't quite figure it out). Perhaps they are much more sensitive to sensory evidence and its significance than the rest of us.

Because my speaking the truth is part of the best explanation of why I said what I did.

There seems to be a pattern of argument here that needs to be questioned. It is: (i) Method M for reaching judgements isn't completely reliable; therefore, (ii) method M can't be trusted. The conclusion simply doesn't follow. Method M might be very reliable, in which case it can be trusted to a high degree.

I doubt "group opinion" is the only method available to us for distinguishing hallucination and illusion from perception. (For example, in the land of the blind, there might be ways of correlating your apparent perceptions with facts on which you can all agree, e.g., that there is a boulder in such-and-such a location that wasn't there yesterday.) But it might be quite a good way of drawing the distinction even if it isn't a perfect way.

I assume your worry is not about whether you are untrustworthy in some areas, or in some sorts of enterprises. As Peter Lipton says, if you are dishonest as a general rule, then plainly you can know that about yourself. And all of us have excellent reason to think that we are not good at many things.

But if your question is whether you could have any good grounds for thinking that you are untrustworthy in some very general way (for example, epistemically--in the way you generate beliefs about the world), then I think the answer is also yes. Most skeptical arguments seem to me to give at least some reason for thinking that we are epistemically untrustworthy. Most philosophers these days are not won over by skeptical concerns, but that is not to say that they regard such concerns as logically impossible or incoherent. I think, moreover, that the field of moral psychology gives us some reasons for thinking that we may be somewhat untrustworthy in other areas, as well.