My sons were exactly the same as that and I felt more or less the same as you do. It is really hard - you feel like you are always taking the little ones side and never being a very good mum to either of them.
My sons are now nearly 4 and nearly 2 and, for the most part, they now get on really well. They play well together and enjoy each other a lot. Ds1 would do almost anything for ds2 and vice versa and I have seen him stand up to another bigger child who pushed ds2 around on a couple of occasions.
I found that when I had a toddler and a baby, they wanted such different things from me, one wanted feeding, the other wanted playing with, one needed his nappy changing, the other kept wanting the potty. There are bound to be conflicts. Once things evened up a bit and I had two more equal boys, all that hitting and jealousy faded. They still fight every now and then and probably always will but I can stand back a bit more now that ds2 has more of a chance.
I got really proactive with ds1 when ds2 was asleep and got activities out for us, making a big thing of us two doing something together. I also went out to a lot of groups so he could play with other children and toys and it made the relationship between ds1 and ds2 less intense. I also tried to find activities they could do together, hard when your dd is so small but it won't be long before they can join in with some of the same toys.
When the hitting actually happened I could never decide between removing ds1 from the situation, putting him in his room to calm down, or doing a 'feeling sorry for the baby' act - as then it is not him who gets the attention for the crime which is maybe what he is after. It does sound like he is wanting you more than you can offer at the mo what with feeding etc. I also tried to tell him that having a brother was a very special bond, better than a friend. Brothers look after each other etc etc.
Don't feel guilty for inflicting a sibling on him. I kept consoling myself with the thought that giving him a sibling was maybe one of the best and most important things I would ever do for him, even though the short term was hard.
I'm rambling now because I am tired but what I mean is...it does get better. I really feel for you, it can be awful.