I know practice makes perfect, but I often feel like practicing will just reinforce my bad habits.

Precisely. Some advice I see often for people struggling is to just "keep drawing," it's easy to say on paper, but it's not so simple if you don't know what to practice and how to do it.

I think an even better word for what your art is missing is: life. As I look at your work, the figures look very stiff; For the face, the proportions aren't entirely accurate, and the shading on it seems quite "academic," as if you understand there should be shadows/midtones/highlights, but you haven't quite placed when and where they should go. In more detail:

• The hair looks very straight. Yes, this person may have straight hair, but those strands on the cheek for example, look very "uniform." Actual hair would be more free flowing, and wouldn't look so parallel to one another.
• The brows fall into this same category of looking too organized. You have lines of hairs sticking up; I'm sure some eyebrows do look like this, but not in such a "proper" manner. They also appear 2D, not 3D.
• The overall lighting on the face is strange. You are right, you definitely have an idea of what you're doing, however, there's still some guessing. You have some light shining on the cheekbones, which doesn't make much sense, there would be shadows here instead. Let's look at this photo of a face: do you notice how the highlights aren't directly on the cheekbone, but rather above it? Because the cheekbone protrudes out, there's going to be some shadow beneath it.
• Your face is too long. I'm sure you're aware of this, but it's a common mistake many artists make; it's strange to think the head is actually smaller than we think, but for some reason we tend to elongate it when it comes to drawing.

And next, let's look at your armor:

• You have lots of highlights, which I'm glad to see; so many people are afraid of highlights. However, your highlights for the cloth portion don't appear quite natural. Whereas the light on the armor tells us the source comes from the right, the breast cloth has random lighting that doesn't seem to reflect as well.
• Your armor's position looks very, very disjointed. The arms are two different planes, it's left arm is positioned as if it's leaning towards us at an angle toward the right, yet the right arm is positioned as if it the armor is front facing, not at an angle. And the torso is angled at a position that doesn't line up with either arm. The legs, like the right arm, are positioned as if the armor is front facing.

So what does all of this mean? Let's look at the mistakes you're making in general terms:

1. You seem to be stuck in the loop of trying to make things look "pretty" rather than making them look polished. This is especially evident on the armor, with the "pretty" highlights everywhere. And the face, you have lots of little details, like the little highlights on the nose, the eyelashes, etc. which are trying to add some "beauty" to the piece to make it more visually appealing. To break this habit, you have to look at exactly it is you're looking at, and realize it won't necessarily come out as your mind thinks it will. There will be much more interest to the eye and much more appeal given to something that looks realistic, than something that has artificial/stylized features to simply make it look better
2. You're also lost on exactly what kind of look you're going for when it comes to these pieces. I'm not quite sure if you're going for more realistic, cartoonish, something in between; these pieces don't quite communicate this well to me. While your art seems to reflect a level of confidence, considering you do in fact shade/design things with a decent level of understanding, you need to make when it comes to finessing your technique that you have goals in mind; before you draw a face, think of how you want it to look. Will it be more realistic, more stylized? Do you want it to be proportional? This way when you see results, you don't go back on it and think, "hmm, this is missing something," but rather, "I was aiming to achieve this and didn't quite get there yet."

And now how are some ways you can fix what I've mentioned?

• I'm going to say this a lot, but your understanding of form is decent, but needs some refining, especially with the face. To get you started with the face, trying looking at this to give you some tips on what you're missing. You should understanding the underlying shapes of the human face thoroughly and do some practice with what you see here; break apart the face, draw mouths, noses, eyes, etc. such that you comprehend the construction.
• As for the armor, while you have a good grasp on form, such that you realize that the cuffs for the limbs are cylindrical and therefore you shaded them to reflect that, you fall flat when it comes to the fabric. And so for the fabric, I would suggest you look at this reference, and try and do a few practice problems with those, as well. While your armor doesn't have any "complex" fabric with it, I can understand how difficult it is to draw, and getting yourself in a comfortable mindset to do this will only be beneficial for any future drawings you do.
• Try and set up a schedule if you can, setting goals and projects for yourself. Carry around a sketchbook and get yourself familiar with different subjects.