Keep in mind that while you are working, chasing after your two year old, and taking care of your home and spouse, your body is also creating a brain, central nervous system, digestive system, blood supply system, organs, limbs, etc...
No wonder you're soooo exhausted!!!!
Listen to your body - Take a break - sleep whenever possible and throw guilt out the window. By taking care or yourself, you are indeed doing what is best for both your children! Ask your husband to spend more time with your son, and maybe suggest some things they could do together. While they do those things, sleep!! Your body needs it!