First of all, congratulations!
Second of all, if you read all the posts on Mamapedia about women feeling guilty for having a single child and depriving him/her of a sibling, you would be amazed. You have the opposite problem - you feel guilty for GIVING your child a sibling. My single child always griped that he didn't have a sibling. So you can't win if you think this way!
What makes you think that children benefit from getting ALL the attention? What makes you think that they shouldn't learn to share and have a sibling as a friend and being part of a happy family?
You can't imagine loving a second child as much as your first, perhaps. That's very common. But your heart has the capacity to love more and more. You will not love your toddler less when you have the baby. You will be a bigger family. Yes, sometimes a 3 year old gets ticked off when he doesn't get his way, has to share, has to follow the family rules. That's what 3 year olds do. It's also what 5, 9, 13 and 17 year olds do. That doesn't mean it's wrong to have another child.
You'll figure out ways to have him be the big brother. You'll figure out ways to juggle. You'll figure out ways to address his concerns. Not having a 2nd child that you want very much is not the way to handle your first child.
It's time to start enjoying the pregnancy, and to realize that your hormones wreak havoc with your thoughts and focus and emotions. You will have these thoughts from time to time, but you don't have to give in to them. Just ride the wave and you'll arrive safely on shore!