Are you getting help for your ADHD? I think medication and coaching can really help with some of these problems, especially the ones related to concentration.

If that's not an option, have you tried tutoring? A lot of high schools and colleges offer tutoring programs, which can help a lot with understanding material. It's also nice to have small group or one-on-one time with someone looking at a topic. It makes it easier to focus than in a lecture setting.

You should be able to get test accommodations, such as a longer period to take the test, or a separate room with fewer distractions. Ask your guidance counselor if you're in high school, or the disabilities office if you're in college about it.

You're not stupid. You're struggling with something that isn't your fault. You're not stupid. It takes twice as much effort for you to certain things than other people. That's ok. You just need to learn how to work around it. You are capable of learning. You're not stupid.