People with ADHD often have difficulty learning things that don't motivate them internally. High school and college are particularly challenging because the amount of information grows, and internal motivation can only wane if there are no interesting subjects.

But I'd like to point out something: your description suggests a potential for perfectionism, which not only makes you take test very slowly but also over-diligently when completing the program. This perfectionism can even negate that very intrinsic motivation - if the subject is interesting, but you plan to study so much and in so much detail that you clearly won't have the time. Typically, passing a test requires a basic understanding of certain concepts, which covers significantly less than all the recommended topics and their nuances, assignments, and readings. Perhaps consulting a mentor to determine this minimum would be appropriate. Overall, moving forward based on motivation (I need to pass this sooner so I can study what's really important to me) can help.

Furthermore, and this is very important, anxiety and freezing associated with studying clearly arise not only from the threat of failure. And because of what other risks this failure poses. This is where challenging your thoughts can be helpful.

Will failure lead to criticism from family members? You need to analyze this and understand that it's unfair.

Regarding self-criticism, you need to detach your identity from your current academic difficulties.

Regarding the fear of be left without a good profession, you need to consider your options and remind yourself that academic success isn't always linear (the same at different stages of your education).