You have been busy. To make the head/faces distinct vary the features: eyes, nose, cheekbones, mouth, Chen, forehead, ears, etc. I have an idea: put these various features on say 3in x 6 in cards. Make as many as you can & keep them in separate stacks. Shuffle each and deal a card from each. From those cards, make your face. You can repeat this process as many times as you want. Just re-shuffle each deck & get a new face. Also, you can go out & about with a small sketchbook & draw the faces/heads you encounter (in a park or mall). Line-of-Action (L-o-A) has a section on heads/faces that you might avail yourself of. You could expand your subject matter to figures. Study human anatomy - both muscles & bone structure. The internet has a great deal of information in this area. Whatever you decide remember: Practice makes better.
Wow! Nicer job on your face drawings, KatieOfConrad. Very slicker, smoother, and has a very nicer appeal to it.
If I were to give you an honest and sincere opinion on your facial expression 2 minute drawings,here is this image below:
I'd looked at all of those sketches, and it states that you're on a totally right path (I believe), why don't you loosen up your recognizability of your sketches in 119 seconds per scribble. It can get so, you'll even get into cartooning and caricatures in those facial expression designs. The reason is because, your recognizability of those facial feature sketches will become more solidest, fluidest, and most energetic.
It's really weird despite how distinct the faces appear to be, your noses seem really samey. (Mostly when you have a drawing where you're kind of looking down on them a little) Even though you've got some very distinct and different nose shapes on some of them. If I were you I'd try to do a couple of nose studies maybe and see if there's a marked improvement. I like your lines and for two minutes you got in an impressive amount of detail.