i think the emotions your having are normal, i certainly went through the "how will i cope stage" but i was also upset as DD1 was only 15mths when we had DD2 and that wasn't part of our plan.
That said my girls get along very well, even at 15mths DD1 was so intrigued and willing to help.
There is no right or wrong way with two, you very quickly learn what will & wont work for you, whether it be settling bub or toddler first, who naps first etc etc.
You will probably find once #2 is here you will be a little more relaxed & at ease about the baby stage than you would have been your first and in regards to introducing someone new to the relationship think of it as a new experience and way to spend time & teach your toddler - babies really are a great learning experience for smaller kids