How do you let go?

People always tell you that if someone is hurting you badly you should let go, but what they dont tell you is how to let go. Lets be honest if you've loved this person for over 3 years and you knew all along that he ain't acting right but you still love him anyway. But you've reached this point where you can't take it anymore, you've been broken so many times and you believe that you can no longer love him because loving him means breaking yourself more each day and you'll become your own destruction because of this fatal one sided love. Shouldn't the red flags have made themselves noticeable a long time ago? And even if they were noticeable, did you purposely ignore them? When do you draw the line on the amount of hurt you will take? There are times when you honestly dont know why you love him, but you see him and all doubts flow away until he leaves your presence but stays in your mind to haunt you with "whys" for another day. How do you know when to let go? Why does everyone make it look so easy as if you just say bye and Bam! He no longer lives in your mind, you no longer remember the touch of his skin or how his words made you and broke you at the same time.  Someone tell me cause I really need to know, when do you let go?