you had a traumatic experience of losing a child almost 2 years ago. besides the trauma of losing your child, you had to suffer physically too. the trauma has left a big scar on your soul and you are living the pain since then. it is almost like post-traumatic stress disorder that you have developed. you have not been able to come out of the loss. you are also depressed, it feels.
i can empathize with the loss. but i am a little surprised at one thing. you didnt mention anything about your feelings and behavior towards your daughter. why is this anger and frustration mainly towards your son? is it because he was there when you had the loss (do you unconsciously feel that it is because of him that you had to abort the other one? that he was small and you couldnt have another child right then?) correct me if i am mistaken here. but the anger may develop when we abort a child for the sake of or due to another child. its ok to feel angry. but not ok to vent it out on the child who is helpless. your son is suffering because of all this. and you need to take steps right away.
good things is that you have already taken the first step by realizing that there is a problem somewhere. but there is no one who can help you 'be happy'..happiness is something that comes from within. our reason to be unhappy lies somewhere deep. unless that is dealt with, happiness will always be dependent on external world and will be short-lived.
you may want to work consciously on your frustration tolerance. for that, you need to de-stress yourself. check your daily schedule and see if you are overburdened with household chores + managing kids+ managing husband. you may want to take out some "me-time" for yourself. involve your husband in taking care of kids and take a short break once in a while. for more details please check these links
one thing that helps the parent stop/reduce beating is, 'everytime you feel like beating the kid, look at him and imagine him telling you "i love you mumma" with his big innocent eyes, imagine him pleading you "mumma please dont beat me". if this doesnt help, immediately go to another room and bang into a pillow/wash your face/vent out in front of mirror..go to your child only when you have calmed down.
the guilt of killing a baby can be over-whelming. it is not something that can be erased easily. seek professional help if required.