I would suggest trying to just start simple and worry about details later. First focus on drawing the background, midground, and foreground shapes. Once you create a good sense of depth then maybe focus on details like trees & clouds etc. I find the same helps me when drawings characters also.. first focus on interesting shapes/silhouettes and worry about details later.
Every painting I do... I learn a bit more. I too don't use any custom brushes. I'm trying to paint with oil on a computer screen I guess. I just use a round brush and change opacity and density for the most part. practice practice practice is right.
The main thing about drawing anything is knowing the subject very well.
If you want to learn how to draw one kind of tree you need to study it, but start in a distance so you learn the basic look without a lot of details, just its basic growth pattern, colors, and effects like lighting, seasons, and weather.
You will need to practice drawing it a lot to know what pattern and brushes looks best.
Here is a drawing I worked on to quickly do patterns. I started with drawing grass with brush strokes as a pattern for grass. I did some moss, tree bark, leaves on the ground with just stroke patterns to make it look right. It don't take long after you know what looks right, but that takes practice, with a lot of trial and error. You need to learn many ways that don't look right just to find the one that looks great. And it develops the skills to move in odd patterns that do look right.
After you know a lot of patterns you can create things that look great in a sketch that looks finished, because it will be that easy. So, start with something you know you need in a scene like mountains and trees, and start to learn the basics of drawing them.
Pick interesting locations, sit, and then draw or paint them. Also, if you have a camera, take pictures of locations in interesting angles and at multiple zooms. Not saying you have to recreate a photo, but locational photography can help greatly with how to map out objects in a compositions, cropping/framing, depth of field, and lighting when it comes to landscapes. Look at your photo and find these things. This will build your eye and technical execution.
I've struggled with this a lot as well, what I've tried is to start painting abstractly by laying colors down then going through and refining objects with highlights and shadows. I think I'll start one now, I haven't really ever done a heavily landscape based painting. I'm always so amazed by Bob Ross, well still I should say. It's good advice to just keep practicing that one thing until you're good at it or confident at it I should say, that's what I did with painting noses.