Whenever I get a new story idea, I usually think on it for awhile. I percolate on it for a few days before even talking about it to see if I actually like that idea and if there's actually something there I'd like to write about. Once I've done that then I'll go to my sister.
I'm sure she'll vouch that we will spend hours going over details of whatever story idea I bring her. She helps me think of things that I normally wouldn't and really get a better understanding of the story and characters. She's actually the reason why our YA story is no longer just an idea we've discussed, but now in the process of being written.
Anyways, my sister really helps me plot and plan out any idea that I have. It's something, as writer's, that I think we all do at some point. Even if it's just a character outline or an overall guide to help you with points you want to cover, we all have some form of outlining and plotting that we use.
No matter how much planning we do though, there will always be some point that we overlook or decide to change as we get into the story. And that is perfectly okay. In fact, I'd find it almost impossible to write without that happening at least once or twice. Having your characters and story grow as you write it seems like a natural progression, like that's the way it's supposed to be.
So, in essence, I don't think you can have a PERFECTLY planned plot. There will always be something you'll want to change, add, or get rid of as you go. BUT, I think you can strive to have a plot and story that's perfect for you. Something that just feels right: the characters, ideas, flow just all seem to come together in a way that you love and can't wait to see come to life on the page.
We all strive for that moment. And stressing over the little things about getting there will only make it that much harder. I say enjoy the journey and let everything else work itself out.