Why do I keep blaming myself even though he treated me bad?

I was with my sons father for over five years. Our son is 2 and we broke up six months ago. When I was pregnant I told him I did not want the pregnancy to be the reason we would get married but I would want us to get married so we could be a real family and he agreed that we would get married but not right away. Well that never happened and he made it clear many times that he had not intention of ever marrying me. A few months after my son was born he went back to the way he used to be. He would talk down to me(even in front of people), tell me it was my job to clean the apartment(meanwhile I work full time and went to school), that i had to pay for more of the bills. he also never showed me any affection unless he wanted to have sex. I thought if i did certain things it would make him love me and care about our family. I was wrong because he started spending money on a boat and other things instead of making life better for him family. I will not say he is a bad Dad but he is a Dad when he wants to be, the other time he does what he wants. I got upset and angry at his behavior and I can say I must have become that annoying nagging girlfriend. All I wanted was to be cared for and appreciated for all the things I did. Towards the end of our relationship he did not invite me to a party that all other couples me know where at. So I sat home with our son sleeping while he went and later I find out everyone was there. It felt so hurtful to try to make a relationship work and the person doesn't even want you to be with them. I finally left and I have to talk to him due to our son but it makes getting over the breakup harder. I know he is going out having fun while I am home usually with our son and he can be a Dad when he wants to be just like before. Now I saw he is on a dating website and he says "he is looking for someone that he can mesh with" I know he was not good to me and I need to move on but thinking that I wasn't good enough and the is looking for someone else really hurts. Especially when I tried so hard to keep our "family" together. Why do I keep thinking maybe if I didn't so this or that we would still be together? And my son would have a "family". Why do I care if he wants to meet someone else when he treated me so badly?

we always blame ourselves for others failure, especially ones we love. Sometimes it turns out the love we had wasnt reciprocated, and sometimes sadly it takes time to find that out. You sound like a very intelligent woman, and a loving mother. I can tell you to wake up, but you spelled it out yourself, and if you need to hear it from somebody else, i'll be happy to say it...He didnt deserve you, he didnt deserve your affection, and he was so far out of his league in the world of real people, that, well, thats why he is on a dating site hoping to "mesh" with somebody, and your seeking emotional/relationship help... Its really hard to be the lone fighter in a loosing fight, because no matter what anybody says, you cant quit fighting, and then when you lose, you blame yourself. Maybe if i had done this instead of that, or said this instead of that. Normal insecurities, but you as a fighter, i think you can rise above it, might take some time, but you know what im saying. You did it right, even if nobody told you that, you did, and i dont know you from Adam. Take care of your son, and then take care of yourself. You are stronger than you think. You still have feelings for him on some level, why you care if he meets somebody else...even the Truth cant explain that one in less than 1000 words, but its very normal, and i KNOW you will get over it, the time it takes is up to you...and fighters are quick learners!