Hi,
Welcome to the forum, and to the scary world of TTC :). I think your sister is quite right that most people feel some ambivalence or fear when they start trying to get pregnant, or when they find out when they are pregnant. The same thing may happen when your kid starts school, or when your kid starts being interested in romantic relationships… and so on, and so forth. Having a child is a huge responsibility. It requires, I think, a big leap of faith. And then we just have to plug on and do the best we can. We never know the outcome in advance, and we've never been as invested in a good outcome before.
That kind of fear basically takes care of itself. When you are thrown into the deep end, you haven't got much of a choice but to swim. That kind of fear is natural, and will work itself out through practice, until we reach the next Big Difficulty.
But your fear could also be caused by a genuine feeling that you do not want a baby (at the moment). If that is the case, you should listen to your innermost voice and get back on birth control, though not the same one by the sounds of it.
I wish you all the best in finding your way.
Olivia