I think i am kind of mentally retarded?

[Wow]

No, I don't think you are mentally slow or anything. It's possible that you may have a learning difficulty / learning difference. And you seem like a very sensitive / emotional person which can a result of low self esteem due the frustration you feel about your unknown or undiagnosed disability.

I felt the same for years and years, I then went to University and found out that I have severe dyslexia and dyspraxia. Things improved after that. I learned that I was very intelligent in other areas that the average person would struggle.

You are probably not as slow as you think, your brain might just be wired differently and that you are having to function / do and learn everything like the average person who has no disability.
We are all differently, some are just too different though. Does not mean you are stupid, inhuman or anything.