not focused and ditracted

im 22 girl..its started at high school
i lost attention in the lessons i cant focus with the teacher ,i dont know what he is talking.just distracted and thinking empty
only study when i return home and get b+ in my exams , graduate highschool with A- , i entered engineering college (i still dont know why i done that) but still
i try to hear and focus but i distracted quickly
you can say ..i just walk to college and return without any knowledge ..i just study at home
by the way i also distracted when i study for the exams or using net for some research
its like just loosing the time for nothing
my gpa at first year in college was 82% ...im now at fourth year and my gpa is 77 %
by the way i did not fail with any course in my life (yet) i had different grades in courses 60%,70%,80% ,90%
i feel like the dumbest human on the earth but study engineering wow
sometimes if i didnt understand something in some course i dont ask teacher or students for help because i feel like an idiot
so what i must do

by the way i forget a lot ...i make some blood test and check b12 but was very good