Why is it my child that always pays?

My name is Litia. I'm starting this blog today, sort of as a diary for the whole world to see... or not see. Call it a means to cope.
I have three absolutely amazing children. My two oldest have been solely with me for eight years now, their father is not in the picture at all. About 6 years ago I got involved with a man I had known all of my life... or at least thought I knew. He is a psycologist, and the father of my youngest child. I admit that at that time in my life I was lonely, tired of doing everything on my own and faulting myself for the fact that my oldest children did not have the "joys" of having both a mother and a father. I worked a lot for little money, spent time with my children when I wasn't working or going to school, and occasionally went out on dates just to try to gain some "me time". I was casually dating a guy for maybe two months or so before the psycologist called me up one day and announced he was comming to visit and wanted to know if he could stay with me. I haden't seen or talked to him for at least 3 to 4 years and always carried a torch for him from the time I was a kid, so I of course jumped at the chance of having him stay with me finally. Looking back it's sickening how "available" I made myself to him. He came into my life with all the perfect words and actions, telling me how proud he was of me that I had done everything on my own for so long, even though I was so young, telling me how he always thought I was so beautiful but never thought he could be with me or be enough for me. I fell so easily and nievely into his dreamlike fairytale of how life would be if I were with him... and yes I was completely in love with him and had been for as long as I could remember. I became pregnant the first night we ever spent together. I found out a few weeks later, expecting him to be as happy as I was, but instead he accused me without accusing me, that I had cheated on him or that there was no way it could be his, so he threatened to leave me if I didn't get an abortion. I went against everything I belived in, everythig I stood for and had it done so that I could be with him. Truthfully, I still hate myself everyday for making that descision because everyday I have to deal with him reminds me of what I gave up in order to be with him in the first place. Two weeks later I ended up pregnant again and told him, " I don't care if you leave me, obviously, someone out there wants me to have this baby." As soon as I said that he said he was so happy that I was having his baby and he wanted to be with me but he lived three hours away and he didn't want to leave his mother who was dying of cancer. So I offered to upheave my family and come there to be with him and said to him, " If it were my mother or grandmother I know you would move here for me." How stupid I was! Needless to say it didn't take him long to show his true colors. First, he found me a car which he put in his name and I paid him for it when I recieved my income tax at which time he was supposed to put it in my name and just kept putting it off, saying things like, " why do you need to have it put in your name, are you planning to leave me?" Then he wouldn't let me work and told me, " You're pregnant you shouldn't have to work, " or " Your to sick to work", which soon turned into, " If you go to work you might meet someone and leave me." When I was pregnant he was in school for his psycology degree and he would constantly try to diagnose me, telling me I had this disease or that mental illness. When we would fight he would tell me how things that I did was the reason he was acting the way he was acting and how I was trying to mind-fuck him into doing things that I wanted. After I had my son things were good for about two weeks, then I told him I wanted to go to massage school because I wanted to make a future for myself. He didn't see any reason for me to make a future for myself and said that he was my future, but I wouldn't back down this time. When my son was four months old I started school for massage, I was breast feeding my son and would go to school in the evenings, feed him before I left, put him down for a nap, then feed him when I got home. From the time I started school I would come home and every night he would accuse me of cheating on him with my professor or someone else. About two weeks after I started school my son ended up in the isolation unit of the hospital for a week (they never did find out what was wrong with him), I called my mom and had her take my girls so that we could be with my son. Instead of being with his son at the hospital he called up his best friend and had him come up for a week to play video games online and only came to the hospital twice to see his son because I asked him to so that I could get my school work, wash my sons blanket, and take a shower. Our son was released from the hospital, but had a follow up appointment with the docter a week later. I went back to school and again was being accused of cheating on him or planning to leave him, he shut off the electric in every room of the house accept his and took the battery packs out of all the phones so I couldn't contact anyone. I remember I was sitting in the bathroom in the dark dryheaving into the toilet because of how upset he was making me and he stood over me and in an eerily calm voice and said, " Do you see what you make me have to do to you to get you to listen to me?" I came home from school the next night to find him and my son gone and my girls (who called him Daddy) home alone. He renewed the lease on our appartment but didn't put my name on it and gave me and my girls 24 hours to leave his appartment. It took me two months to get my son back and because his dad is a criminal justice major and a psycologist he never even got a slap on the wrist for taking off with him, the custody concilliator said that he was just going through a lot of stress with his mother dying and loosing me and the girls, when itwas him who kicked us out! So we got 50/ 50 custody and over the years he has done the worst things to our son in order to try to hurt me. Last year I moved away to be with my boyfriend and lost custody of my son. I now only see him every other weekend. After that hearing my boyfriend and I decided that we would stay in our house and fix it up over the next year or two then move back to be with my son. Recently my boyfriend decided he no longer wants to move to be close to my son so I moved back to my hometown with my family to save some money and get the support that we need to move back out to be with my son. Now he is telling me that he is withholding my visitations unless I give him information that he wants about my life. Why does he keep doing this to my son? How does he keep getting away with this? I feel so alone here. Why is it my child that always pays?