Be careful with textured brushes, I only recommend using them if the rest of the shading is textured or it fits with your current shading type! Shading with a heavily textured brush in areas with smooth marks can make things hard to make out or distracting as it draws focus to the areas.
Also be careful with the colors used for shading and using black/white as shadows or highlights! It's a really tricky thing to do, it can fit with certain lighting but it can also make the art look muddy and uninteresting. Deciding on a light source and adding contrast with the backgrounds can help a whole lot with this! Think about what color your lighting is, if it's yellow will have a golden highlight, blue will have a blue highlight and so on.
Hope you don't mind- I'm going to redraw one of your panels to help explain some things I can't quite describe entirely.
First of all, clean lineart is everything! It can do so many wonders. Lineart is something that you need to practice as much as you can, but to help you many programs have stabilizers that smooth out your lines for you. If you're using photoshop, I believe there's a stabilizer extension you can download.
It seems like you want to go with a slightly shaded lineart look, so I filled in some of it. I'd stay away from overusing this, like filling in the entire perimeter of the face with the black shadow, so I stuck to one side. Keeping lines clean will also make it easier to distinguish what exactly you're looking at.
Next I added the flats, make sure to keep these on a separate layer from your lineart! Keep them separate for the entire process as well, merging them can make it very unnecessarily complicated if you have to go back and edit anything. The colors themselves were color dropped from the panel itself. I also changed the background color to maybe fit the mood better, the blue seemed out of place and I couldn't tell if they were indoors, in the daylight, etc. I just added a gradient with SAI's marker tool.
To begin the shading portion, I made a new layer, clipped it to the flats (so it stays in the panel) and set it to Multiply. Since Multiply makes colors more transparent the closer to white they are, and that SAI's marker tool blends best when there's already color below it, I filled it with white as a base. The white is completely invisible and has no effect on the colors.
I filled in the base for the shading with a mid-light grayish purple, I blocked in the areas that'll have the darkest shadows and will use SAI's marker tool to blend it out. Since you had the highlights more on her left side I've placed the light sources there.
This process will vary from program to program, but Paint Tool Sai has it's own tools that blend based off pen pressure. In programs like GIMP or Photoshop, you will probably have to blend it manually with the pen tool by changing the opacity and hardness of the brushes. It'll take longer and take some time to master but it's doable and you can probably find dozens of tutorials for it on youtube. You also want to remember that hair falls in strands, not clumps! Unless your character's hair is very matted keep it as smooth as you can.
Here's the base I blocked in for the highlights, since the other panels seem to have a soft yellow light for the source that's what I'm going with. This layer is set to luminosity, but in programs like Photoshop it may be called something else like Hard Light or Soft Light, etc. whatever you feel is the best intensity for it. Since the light source seems to be coming from her left I've placed the majority of the highlights facing that way.
Like the shading layer, I blended it out. Same basic steps.
This is just personal taste, but I added another layer on Overlay and filled in a pale yellow and dark purple/blue with a very soft low opacity brush to brighten the colors and make it more dramatic.
just to top it off, I made the panel border black and put it OVER the character (this is important! You want it to contrast with the character or it won't pop right!) and added another highlight layer on luminosity and painted over with a mid-dark yellow (so it doesn't overwhelm the colors) with a low opacity soft brush. I also fixed some areas that looked a little off.
I'm a little rusty with blended shading with lineart, so please excuse any weirdness with it. It seems like you want to have a bit of a painted look to it so I mostly focused on that. Your style may also fit really well with cel shading, so I'd experiment with that a little too!
Overall your pages have good composition, you have a good grasp of variated angles and camera shots! I'd mostly just work on keeping your lineart clean and then practicing lighting some more! The advice above for word bubbles is also something to keep in mind. Contrast is very very important.
