It's ironic how as authors we can be terrified of our own books. Sometimes the fear comes before we've even set ink to paper, or fingers to keyboard. Maybe it's our first book, or maybe it's our fifth. It doesn't matter. There can always be a bit of apprehension with any new project...but even more so when you're starting out with your first book, or a new genre to you.
You wonder whether you should just dive on in, or if you should outline. Know this is a personal decision and you have to go with what works best for you. Typically you have been granted with the image of a scene, a character, or dialogue--maybe all three--and you know you have to write the book. Don't get weighed down in concerns, just write.
I know some authors who don't even write their books in chronological order. Scenes hit them and they write them. They fill in the book as they go along. (The thought of that scares me, but it's what works for them.)
Some authors outline in depth, while others just go with the flow (panster-style). I find that a balance is most useful for me. While I have written novels just going with the flow, there is usually more work when it comes time to edit. Plotting, even a little bit, may save precious time when it comes to editing later on.
What if you've written your book and want to make some changes? That can be daunting. I know I've feared touching my manuscript before worried that I would ruin the whole thing. The thing I remind myself of is the book is my creation and I can't go wrong. With this said, I strongly recommend you back up the original and save the revised one with a different name. Then, if you're not happy with the changes you've made, you still have the original to go back on. I usually also put the "track changes" feature on in Word for larger plot changes so that I can review again before I approve.
Either way, as you can see there is no need to be afraid of your manuscript, whether you're just starting it, writing it, or revising it. Just write and enjoy.