This is going to be one of the hardest things you will do. Ever. Recognize that, and just do your best to get through each day to get home and see your baby. Pumping is a pain in the neck, but an amazing thing you are doing for your baby. Be gentle with yourself and lower your expectations. See sentence 1. Going back to work is a necessity for some, and a choice for some, and always a difficult adjustment. Still, it helped me to get back a part of myself that I had forgotten during those long, exhausting first days of my daughter's life. But no question, it's a struggle. So be kind to yourself, outsource everything you can, accept help, and try not to let any guilt take you down. You're going to feel guilty leaving your baby. You're going to feel guilty if you have a partner who is getting less and less of your time. You're going to feel guilty to your employer for being a frazzled mess and not performing anywhere near your formerly competent self. You're going to miss your former self and wonder where that put-together lady went, before all of the washcloths and burp cloths, diapers, and bottles.
But you just created a human being! What a truly wondrous accomplishment! And one day, not too far from now, you will start to feel like yourself again. You'll remember what it's like to have a quiet moment away from crying and get wrapped up in your work. And then you'll have a pang of guilt as you remember your child, away from you. This is all part of the process of incorporating this amazing human being into your life. You'll grow to a new normal, one where you may never quite achieve the high standard in your mind of the ideal wife, mother, or employee. Just strive toward that ideal, and you'll come closer than you know.
So cut yourself some slack in this huge new transition. Let each day unfold without overwhelming yourself, take care of yourself when that inevitably happens, and embrace the joy that comes your way.