My students really find the simulated diffs on this website to be extremely helpful. A subscription costs $35, and you can choose regular diffs or advanced diffs. The website is
https://www.labce.com/ and you want the "WBC Differential Simulator". As to distinguishing reactive lymphs from monos: there are lots of ways to describe the differences, but in the end, it just takes seeing lots and lots of cells. Eventually your mind will make neural networks and you will just recognize the cells. To me it's like the experience of signing up for a class with people you don't know. At first, you have to make an effort to remember names and recognize faces. You might get two classmates confused. You might have to memorize distinctive characteristics: glasses, hair color, etc. Eventually, though, you just recognize them--even if they change hair color, or wear contacts one day. Your mind is designed to learn to recognize things by their appearance, so relax and give your mind as much experience with seeing these cells as you can possibly manage to give it. Keep looking at cells even if you are sick of it. You will eventually learn. Also, everyone learns at a different pace, so be patient with yourself and stick to it. Lab CE will help--I'm sure of it!