I'm so heartbroken....

So,I started dating this guy when I was 15. He was my heart, my soul, my everthing. I was so inlove. He left me. And dated someone else....3 years went by, and he somehow came back in my life. My heart was whole again. I felt complete. We has a baby a year after we started dating again. He never came to appointments. The whole time I was pregnant, he was never ever home. After our son was born, nothing changed. I've now been raising my son on my own pretty much for almost 3 years now. We have been together 4 years. Everything did a huge uturn. Now, hes out till 7 in the morning. Sleeps all day, wakes up, and leaves all day and night. hes never home. he never wants to do anything with me. He's done so many things to break my heart, but for some reason, I just can't leave...... I cant get myself to just say okay, u need to go. He has no job. he dosnt pay for anything. I know hes getting a free ride, i know hes here becuase of our son, and cause hes getting everthing handed to him. hes always signing up for single people websites, saying he has no kids, dosnt have a girlfriend. Alaways talking to other girls and hitting on them... I just cant come to, and say bye. U gotta go. I dont know what to do everytime i go online, its something new. My heart is breaking. Im falling apart.....i dont know what to do. someone please help.......Im so stressed out. I cant handle this no more....why cant i let him go?????? why!!!!!!! i need help. I need streingth. I need something to hep me through this......whys he doing this to me? why has he changed? why is he always like this to me? what have i done? i pay for everything he needs i take care of our son. I go to college so i can support my family.......why am i such a bad person............why cant i just say BYE!!!!!! :(:( :(