It's ok to be scared, but don't let it run your life. Fear and stress are not healthy for a pregnancy - just be calm and relaxed and everything will most likely be fine. If you really want the kids, and your doctor has cleared you to try again, go for it. You'll be happy when that healthy baby rolls around.
If you had the first one naturally, don't let any doctor assume that you need a c-section the second time around. Were there complications during the birth to make him say that? Or is just because the baby was so big? if you did it once, there is no reason why you shouldn't try to do it again next time.
If you do end up with a c-section, don't worry. They really are not that bad, especially when they are scheduled. It is actually easier in some ways, since you're not exhausted from labor.
If the doctor is telling you now, before you're even pregnant, that you have to have a c-section, honestly, I would switch doctors. That is just stupid.