A few observations:
1) It is hard, and it will get more challenging with a newborn. 2) It will get better, but probably not soon. 3) Get help. Can you hire a mother's helper for a few hours a day? 4) Get a good sling or other babywearing device baby a lot so you can still be mobile to meet the needs of your toddler. 5) Childproof your home as much as humanly possible so you don't have to be on top of your toddler every second--even if that means more childproofing than you would otherwise think reasonable. What is a reasonable degree of childproofing for some children may not be adequate for yours. 6) Two year olds absolutely *do* understand consequences. They just have to be immediate and very, very consistent. Two year olds also are perfectly capable of learning that you are too tired or unwilling or unable to deal with certain behaviors, at which point they know they've got you over a barrel. 7) You must not think there is nothing you can do. You probably can't make him a more laid back, slower paced child, but you absolutely can teach him some boundaries to his behavior. It may be more challenging than with many other kids, but it is doable. You may just need to choose your battles carefully and be very, very consistent.
Best wishes, Ericka