I feel your pain. I've always been awful at drawing, but here I am, needing to create pixel art.
I could try suggesting a few things. There were a few courses I went on that really helped. One is on Udemy, but if you hunt around for a voucher or pick the right time then a course is only about $10-20.
https://www.udemy.com/pixel-art-for-video-games/The author is also an illustrator, and the course is jam-packed with information about illustration in general and pixel art in particular. I felt like it was, in a small way, teaching me to "draw" through the medium of pixel art, rather than just giving me pixel art tips like other courses and tutorials. It also had a section dedicated to the kind of art you want to recreate. (Mainly trees, rocks, and character design). The author also has a Youtube channel.
But if you've tried a lot of tutorials and they're not helping then you may be just throwing money and time away.
So, things that have helped me that I can suggest:
Challenge yourself. You need to be practicing all the time. You'll only get better by doing. Pixel Dailies on Twitter is good.
Learn how light works. This is the most important thing, IMO. In order for something to look convincing, the light needs to work well enough to trick your brain. This is how you create solidity, depth and surface texture.
Study the world around you. If you're learning how to draw, you will start to see the world differently. As you walk around your neighborhood, take some time to look at how the light plays on a leaf, how leaves arrange themselves on a tree, how a fence or a brick wall change in color, shade and texture depending on their angle towards the light. Etc. Etc.
I would say keep looking at tutorials. This guy made a lot of pixel art tutorials as pixel art, all available on the blog for free:
https://blog.studiominiboss.com/pixelartThere are hundreds more around.
Personally, I started making progress when I started studying other pixel artists. By working out how they did things like pick colors, represent depth and apply shadow, I started to get an idea what was going on. Pick a distinctive artist and ape their style! Or find game art you like, study it pixel by pixel and draw your own takes on the ideas you see.
Another thing to try is recreating photographs. I found this to be a really useful exercise: take a photo that you want to make into pixel art and reduce it down to (say) 64x64 pixels or something small like that. It will be a horrible mess, but you want to just eliminate the detail and block out any large, obvious areas to get approximate proportions right. Now pick a small palette and put the detail back in, using the photograph as a guide. You will have to do things like recreating the "idea" of something at a small scale, and translating the play of light and shadow into a different medium. Both things you'll need.
Or: "cheat." If you don't have the time or desire to learn this art form, you can employ someone to do it for you. I'm sure you can find someone on here or on Fiverr or something. I've done this, and I found it to be a great starting point. I ended up taking it and developing it myself, so it doesn't have to be either/or.
You could take a course in illustration or drawing if you liked! I like pixel art for its own merits though. It's like a cross between art and a puzzle. I have no desire to pick up a pen or a brush.
First and foremost, accept that a week is nothing. You're not going to learn how to do everything you need in a week, or a month. Yes, it's pixel art but it is also art. You wouldn't expect to paint good oil paintings in a week, so why expect to make good pixel art? If you're on a tight timescale then buy some in, either a bespoke commission or an asset pack or something, until your skills are up to speed.