don't do something you will regret later?

Today i recieved my report card, it was, well, really bad.....i was 24 out of 42. Others might say that it's not too bad already but i think i should have done better. I should have worked harder, slept less, done more exercise...there are many things that i should have done but didn't do....that's what most people life is about...they say "i should have done this and that" or "i shouldn't have done this and that" after something results from their actions and they regretted it or are not too happy about it. Why is human nature like this? They know what they should or shouldn't do but they just don't do what is right. They know they would regret it later but still they follow their impluse of the moment and sought to regret after that. Why, at many times, your heart tells you what is right but you just can't seem to obey it? *Sigh*
God, i am someone weak of heart, mind, and will. Please help me to do what is right, i mean really really do it, and not just think of doing it.....>_<