How are you supposed to deal with the guilt of your partner having a really hard day at home and you're stuck at work?

You are both adults. You are literally accumulating funds for your family while at work (assuming you are not volunteering your time). He is stressed and, while it is normal to feel empathy for it, you are not responsible for him knowing how to cope with stress. Do what you can from work (listen to him venting if you are able to be on the phone or during a break, order food to the house so he doesn't have to worry about food, let him know what tasks you are willing to do when you get home, etc.). Try not to take on his stress or create stress for yourself by drowning in guilt. Take care of yourself now so you can take care of hubby and baby when you get home.