I doubt that anyone here is unfamiliar with these doubts ;-))

These two doubts are by no means certainty, but your thoughts also seem to indicate that it is possible to be certain of oneself, despite all doubts.

And I think doubt makes perfect sense as long as it makes you mindful and careful. As soon as they inhibit and paralyse you, they are counterproductive and should be actively prevented. When doubts turn into generalised mistrust, there is imminent danger: I see that as a serious psychological crisis...