Do you have any resources at home that you can lean on? A church or community-based counseling service for example? It sounds like you really really need support, and maybe talking to your DH (if you think he'll be supportive) can get you motivated to reach out to someone for help. If you can afford it, getting help with your son during the day or even for just a few hours a week can help you move through some of the difficulties of being pregnant with a toddler.
What you're describing though sounds very normal to me, though perhaps somewhat more extreme. But I had a few weeks (or maybe longer) in the first trimester when I was seriously overwhelmed with what we had done even though we had actively tried to get pregnant and were in the "perfect" place to have a baby. I was depressed and anxious and sick all rolled into one. It got so much better by the 2nd trimester that I attributed it mostly to hormones. But that doesn't make the feelings any less real. I hope you can find some peace soon.
(And on the nausea thing: I've heard people have a lot of success with B vitamins and some have had great success on high protein diets. Does your doctor have anything to recommend? And finally, I had to switch to a prenatal without iron the thought of even taking my prenatal made me wretch. The iron was making me so nauseous. So playing around with prenatals can really help)