I had the same feelings when leaving my first daughter at home for the first time to go back to work. But then I reflect upon myself, growing up with two working parents, and I never did resent my parents for 'not being there'. Children are extremely resilient, and as long as their basic needs are fulfilled, they understand how much you love them.
I also found this excerpt from one of my pregnancy reads very helpful, where the author painted a funny (and rather ridiculous) picture of the "Perfect" parents:
- The perfect parent, if she (or he) existed, would devote herself full time to the care and teaching of her child. She would begin, even before conception by shoring up her folic acid reserves and purging her body of any chemical remotely suspect. Once pregnant, she would never touch a drop of alcohol, pump her own gasoline, get less than eight hours sleep, or allow herself to be stressed in any way. She would have an ideal, unmedicated, and uncomplicated delivery, and breastfeed from the moment of birth until the child was potty-trained. She would know precisely how to stimulate her baby, but also how to avoid overstimulation. She would spend hours every day playing with him - singing, cuddling, talking, massaging, exercising, reading, showing him how all kinds of toys and other fascinating objects work - and never have to leave him in his swing for half an hour while trying to make supper or balance the checkbook. Her house would be baby-proofed, so he could explore every corner and rarely hear "No!" ... She'd start him on piano/tennis/dance/French/swimming lessons at age 3, but if he showed no interest, would happily forfeit the ten week's tuition. She'd send him to the perfect preschool, using their time apart to brush up on the latest child-rearing information and prepare all sorts of new and interesting educational activities for him. And of course, she wouldn't do it alone. She'd have the "perfect spouse" right alongside, equally loving/stimulating/nurturing/teaching their child every step of the way.-
Phew.

If there are indeed parents out there like the above, I'd wonder what their children learn from parents whose only focus in life is their kids. Children need to learn how to work, share, nurture and enjoy life. As others already said, working is part of life and you're not alone!