What is the science behind motivation and why am I having hard time to motivate myself?

I am 22 and a female, and i have struggled to motivate myself for my whole life. I remember being always late to school because i was having hard time to motivate myself to wake up and get ready. I am still always late, having hard time to motivate myself to do my assignments or other daily tasks. Sometimes I am okay for a month; responsible, motivated, awake. But then, again, I find myself craving for motivation. It is really hard for me to have consistent and enough amount of it. My mom has called me lazy, useless, forgetful since I was a child, and I grew up with shame for not doing the tasks on time. When I have good grades it is okay and that motivates me; but one little thing, like a bad grade or anything that makes me feel bad, crashes me and makes me loose my whole motivation. And my grades dropped since highschool because of my motivation's extreme ups and downs. What is the science behind motivation, why mine has been flaky since my childhood, what can i do to need little motivation to do my tasks and to have consistent amount of it?