Your English is perfect. There is no indication of low intelligence from the way you described your issues. B and C grade are pretty average and indicates you can learn with hard work. I think what you might be experiencing is anxiety and that is causing you to think you don't have skills or intelligence. I am not from US so I don't know what kind of help might be available for you there, but I would suggest you to look for a job with something you like to do even though the pay might not be great and then eventually your interest will take you further in your career.
Your English is perfect. There is no indication of low intelligence from the way you described your issues. B and C grade are pretty average and indicates you can learn with hard work. I think what you might be experiencing is anxiety and that is causing you to think you don't have skills or intelligence. I am not from US so I don't know what kind of help might be available for you there, but I would suggest you to look for a job with something you like to do even though the pay might not be great and then eventually your interest will take you further in your career.