There isn’t much difference between certainty and certitude. One definition that certainty does not share with certitude is something that is clearly established or certain. But where certainty means the state of being certain, it is very close to certitude, which means the state of feeling absolutely convinced. If there is a difference, it’s that certitude is a feeling while certainty involves inherent factuality.
Example
In these cases, certitude is a feeling of certainty or an assured manner:
Sometimes an erroneous assertion is made so often and with such certitude that you find yourself wondering how the people repeating it reached their incorrect conclusion. [Boston Globe]
There is an understated certitude about Kelly’s style. [Irish Times]
That war should teach us that self-righteousness and religious certitude are more likely to lead to violent rather than to peaceful resolution. [New York Times]
While a person can have certitude without certainty, certaintyis more solid—for example:
It’s a near certainty that the 17-member nation eurozone won’t survive in its current form. [Toronto Star]
But at the same time this makes it difficult to predict future behaviour with any certainty. [The Australian]
But early repayment charges mean that even the certainty of a fixed rate can entail an element of risk. [Telegraph]