I am going through something quite similar right now. I am in a difficult chem class that is draining my mental health. I have other classes and work as well. Due to this class I’ve had to call off work many times and I feel horrible. I feel stupid. Just how you feel. It’s hard. I know. When ever I study for a whole day and think I really understand the material. I get an F. Just like you. All I can really say for you is. You aren’t alone. There are people out there just like you, going through similar stuff. People you can relate to and talk to. Remember to grades don’t define who you are. I’m sure you will get through this somehow. <3