I'd recommend being resourceful. Don't just stick to one or two or even five textbooks or only just look towards your teachers for guidance. Go on YouTube, look up the content, and watch a video. I'd recommend The Organic Chemistry Tutor (https://www.youtube.com/c/TheOrganicChemistryTutor); he's gotten me through a lot of stuff. Go on Google, look up the shit you're confused in, and focus on the ones you hate and struggle with the most. Granted, good grades don't truly reflect your intelligence, but I think you already know that by now and you also know that a relatively stable, good-paying job will most likely require good grades, so, really, the only way to get your grades back up is to just put more time and work into your academics. There's no other way out of it.

And don't let your low grades now get you down. If it doesn't work out this time around, you can do better next time. There's almost always a next time. Make sure to take advantage of it.

When it gets to this sort of shit, you need to master time management. You need to develop some sort of schedule that'll get your life together. Plan out how much time you're going to allocate to studying and what time you'll spend giving your mind a rest. I, personally, study at least three hours a day. Because I have pretty bad ADHD, I developed a system (it has a name, but I forgot what it was—) where I study for 25 minutes and then take a break for 5 minutes and then go back to studying until my goal is reached for the day or until I finally understand just what the fuck I'm being forced to put up with.

Edit: Remember that when you take breaks, don't do anything mentally stimulating. That means no going on your phone for leisure shit, not even for mindless scrolling on social media. Go outside, take a few breaths, or shove pizza down your throat. Walk around your room. Organize your desk. You're trying to bring your brain back to a level where it won't be overwhelmed with shit to process. Breaks are to calm the mind, not to get it to analyze stimulating stuff.

Also, if you wanna know more, PM me and I'll be able to send you some links that might help.

Edit 2: And remember to not let your missteps drag you down. Mistakes aren't inherently bad; they exist to be learned from. So, go through where you fucked up and see if you can avoid it next time.

Second, I do want to disagree with what ASomeonePerson said above. If you're already stressed, DO NOT force yourself to put more time into studying and work. It might work for some people, but it sounds like that might actually make you feel worse. Instead, maybe come up with a schedule to make sure you have enough time to work and relax. Stress will likely just make things worse, so you need to find ways to just enjoy the time you do have away from work, while also giving yourself time to learn the material. Like they said above, study breaks are important.

Also adding to what they said, I do agree that finding resources online can help a lot! There are even some fun YouTube channels out there that are educational, but you don't really realize it, since they're also entertaining. For example, there's one for chemistry and biology that goes and creates simulations that represent different experiments and situations. He includes any formulas involved, too. Same for physics. So, if you're more of a visual learner, I'd definitely recommend going and finding some of those channels.

When studying, memorization is key. Especially since a lot of tests now are based more on how well you memorize formulas and stuff rather than if you actually learned the material or not. Try finding some fun ways to remember things. For example, here are a few I remember from my classes in high school: we used to learn CHO, CHO, CHONS, CHONP in Bio to remember what elements the macromolecules are made of. Or the dependent variable DEPENDS on the results of an experiment. Or CHina = CHampa (Champa rice; a very important crop at the time of the Song Dynasty). Little things like that can make all the difference when studying. It's been a while since I’ve had to use this strategy, but it might help you if you're having problems remembering what certain things mean. I remember there were a lot of times when I was taking tests and I was saved by those shortcuts. Also, take notes in a way that you'll remember. If you don’t understand what the teacher is saying word for word, don’t write it that way. shortcuts like the ones above can help with notes, too.

Also, maybe try asking your roommate for study tips! What works best for them might not work best for you, but it can’t hurt to ask. Maybe they have some tricks for you that do work. Asking a teacher for help can’t hurt, either.

There are likely some things I’m forgetting here, so I may edit it later, but I'll wrap this up with some personal experiences, because maybe they'll help somehow.

In high school, I was a lot like your roommate. I used to pull all-nighters to finish assignments (partially because I used to procrastinate a lot), but I was easily stressed out by grades lower than a 90 and test anxiety definitely didn’t help. I used to have to study for hours with no breaks to manage all A's, but it got a whole lot easier to maintain those grades once I changed strategy. Again, different people learn different ways, so what worked for me might not work for you, but I started making a calendar with all of my assignments for each class on it, including their due dates. As soon as I got something new, it was on there. Including tests or quizzes. I became a lot less stressed when I saw things constantly being marked off every day. Because that's progress, and you can see that using a list. There's not as much of a feeling of impending doom when you can actually see what you have to do each day. I would finish assignments that were due the soonest first (DO NOT wait until the day before, if you can avoid it). For example, if I had homework assigned Wednesday and due Friday, I would do half Wednesday and half Thursday if I didn’t have enough energy to do it all in one day. And if I finished everything I had due before the end of the week, I moved to things due the next week if I felt prepared. After a while, you start knocking out assignments days before they're due with much less stress and more time to study for them if you need to. This allowed me to take more time to find resources online and study for tests if I didn’t understand things. I was able to memorize formulas, recite notes (slight exaggeration, but I did know a lot of it), and understand a lot more than I did before, because I had more time to do that. The extra sleep also helped me focus more in classes, so I was able to retain more of that information. Basically, the lists helped me manage my time better without constantly stressing over how much work I had. My grades drastically improved after this and it was much easier to maintain them, which is why I’m mentioning it. This could possibly help you relieve some of that stress while also having time to relax and study.

TLDR; I was like your roommate, but stressed and anxious. I changed to a list-making strategy about halfway through high school and was much less stressed, had more free time, and understood everything a lot better.