ADHD does affect focus and attention, in fact the subcategories are ADHD Inattentive type, and ADHD Hyperactive Type (and ADHD Combination Type.,) ADHD affects pretty much everything to do with executive functioning (like emotional regulation, memory, carrying out tasks in order accurately and on time, organization, etc.) I've only been diagnosed as an adult, but I can look back and see how it affected me at different moments in my life. You can start by talking to your doctor about it, they can refer you for a psychiatric evaluation if it's something you're dealing with.