Are Habitual Venial Sins Mortal in Nature?

The short answer is no. A venial sin by nature only “weakens charity” (Catechism of the Catholic Church 1863), it doesn’t destroy it (CCC 1855). However, the Catechism warns us that “deliberate and unrepented venial sin disposes us little by little to commit mortal sin” (1863). So, although habitual venial sin is not mortal sin, it can lead to it.