Is it just that you are either certain or uncertain, or can you be very certain and somewhat certain? If it’s either/or, I know it would make sense to say “almost certain,” but “very certain” wouldn’t make sense. I’m not sure if “sure” is the same thing as “certain,” and what exactly “pretty sure” would mean.
If you are 'almost certain' then you are not certain. So this locution lends no support to the idea of a spectrum of certainty.
'Very certain' as the phrase is used means 'definitely certain'. It signifies that I am not in some other epistemological state, my state is one of certainty - no question. This again lends no support to the idea of a spectrum of certainty. It simply affirms that my state in one of certainty and not something else. 'Very' in other words serves the purpose solely of emphasis.
If I am certain that p then I have the highest degree of confidence that p and there can be no degrees of highest degree - hence no spectrum.