Why do i forget how i was feeling just now

i don't know y but when i get angry at a person after 5 minutes of being angry i tend to forget about the feeling i just had,example once i lent my brother my laptop computer and he drop it and broke it, i was so angry that i almost punched him in the face,so i went up stairs to brood in my room for a while, 5-10 min later i go down stairs and see him in the couch and then i say "Hi Nicky" with a smile on my face,he just looks at me with a confused face.this is not only true with anger but with any other feeling including motivation,i get motivated for a few minutes maybe hours,and then i stop doing what i was doing,the most Ive ever gotten motivated is a week. this is very bad because i did bad in school,so drop' out of high school on the 12th grade(Please don't judge me).a few more things i should mention is that when i was young i had almost no contact with children outside of school,and until a few years ago i always suppressed my anger.
i am thinking if its because of my ADD that i forget my feelings.
this is one of my many problems but that's enough for today.