The number of items up for review can quickly become almost overwhelming if one happens to come back from just a small break. However, in many cases, the number actually consists of a small number of items not fully mastered, and a larger number of fully mastered items that are up for recurrent review.
To overcome the demoralizing effect, I suggest that the system should separate the two types of review items. In the drop-down menu , this could be accomplished by showing the two numbers separated by a plus sign.
Most importantly, it should be possible to select which type of review one would like to do, having separate buttons for each. After finishing the review of one set, the system could then offer the choice of continuing with the other.