Openings are not set in stone
This is the first thing people need to realize when they start a book: Openings can, and should, change if they need to. I have often heard people saying how they can't possibly go on until they have the beginning of the story perfect. And some of them, of course, never go on. Others go on, then find all the work they'd done is useless because something drastic changed in the story. They become disillusioned, seeing all the hard work done for nothing. So some of these people never finish, either.
There are often problems with the idea of trying to write the perfect first scene, some of which are especially difficult problems for pansters (those who don't do any pre planning for the story). They often don't know where the story is going and where it will end, so by the time they get there the opening may not fit with what they create along the way.
Stories begin with a question for the reader. It is not always an obviously stated one, but in general it can be summed up with 'How is what happening here going to affect the character by the end?' This is a question implicit in the opening setting and actions. What is going on? What do the characters hope to gain? Whatever is indicated in the opening has to be settled by the end of the novel, even if it is not the truly most important part of the tale.
By the end of the story, you will also have cleared up any obvious mystery from the opening. If the story starts with someone found murdered, by the end the murder is solved. If it starts with someone wondering if she's going insane, we'll know the answer at the end. The question can be subtle and widespread, a feeling of impending change. In fact, the question, can be anything. The opening scene is going to set up this question for the reader who is going to be asking 'why' from the moment he or she starts reading.
Seems straightforward. So why would it fail?
Because stories, even the ones which are carefully outlined, often morph during the writing. You should be ready for some changes and welcome them when they really do make a better story. And that means you have to be prepared to sacrifice your opening for something new, too.
Say you start the story with the tale of a woman holding on to a ranch. You set it up with loving and perfect detail, introducing her and her problems and then going on . . . but a ways into the story, you realize she really isn't the focus of the story. The true, interesting story is about her son who is torn between his love of the ranch and a longing to be a doctor. You start leaning more and more to the son's POV about life on the ranch, etc. He becomes the main character.
Your opening no longer fits the rest of your story. While the woman is still part of the tale, she is no longer the focus. You might be able to rework it so that the focus includes the son. You might scrap it for an entirely new opening, with the son in some sort of situation. Holding on to the ranch is now a secondary focus rather than the main one and his inner conflict has become what drives the story.
Your opening needs to reflect the core of the story. There is nothing more annoying to me (and quite a few others) than to read an opening about an interesting character only to learn this isn't the person the story is about. Too often I suspect a case where the author had worked so hard on the opening that he or she could not bear to part with it, even when it no longer truly worked.
Now it may be that the extra work you take with the opening is your way of setting the story in your head. That's good and if it works for you, then you need to stick with it. But measure the 'does it work' factor against two things:
1. Do you finish the story?
2. Does the opening still work as well for the finished story as the one you first imagined you were going to write?
You may find the story starts at the wrong place
This is a problem I often have. I start too early, before the real tale gets going. I try to include too much backstory before the true current story gets started. Early history of the character can more easily be doled out in bits and pieces during the story. For instance, in my novel Silky, I had originally begun with his capture by slavers. This meant covering a long ocean journey, sold on the market, years in captivity with a carelessly cruel master . . . but as the first publisher of the novel pointed out, the story truly began the moment Silky stepped forward to save someone's life. Silky even had to tell about his early life to new people, so the backstory was already in place.
It was a moment of true writer-illumination. This didn't mean it immediately stopped me from starting too early, though. Quite often, with the story in your head, it's impossible to tell where to start until you can see it in place. I have discovered a good side to starting too early, though. By the time I do get to the true story, I have a real feel for the characters and situation, rather than that often painful first few pages where nothing feels quite right. I don't worry about starting too early, but I do take a close look and hope, by the final draft, I have chosen the right spot.
If you work too hard you can be unwilling to change later
As I mentioned earlier, a big problem with trying to get the opening perfect is that you may not be willing to rework it again later. And even if you are, think about all the time you've wasted which could have been spent working on the parts of the book that are going to matter.
This can be true at any stage of the story, though I see it more often in the openings.
Don't be afraid to leave things you are unsure about until the end of the first draft. And whatever you do, don't let yourself believe that the first draft is going to be perfect, no matter how much time you take with it. Don't fear or despise editing. It's part of the life of a real writer. Being pretentious and artsy about your work is not going to make it better -- you have to be prepared to work. Imagination is the most important part of being a writer, but it is not the only aspect a writer must cultivate. Without the willingness to work at making your prose the best it can be -- which means rewrites, editing and whatever else might be necessary -- imagination is useless.
If you need to learn how to better write POV, then apply yourself to that work. If you have to learn grammar, make the time to study it. No one is born knowing either and while some people might have a better aptitude for some aspects of writing and catch on faster, they aren't the only ones who have learned.
Be open to change
Yes, I am repeating this from an earlier post. Don't be afraid of change. Don't look at having to rework something as a failure, and don't fear having to learn how to do things better.
Don't linger over your opening more than you need to. Don't let yourself be caught up in the 'perfection mode' when it stops you from finishing the work, whether off the start or later in the story.
Only you can tell the story in your head and you owe to yourself to do it in the best way you can. That means acknowledging that you might make mistakes and fixing them. Learn the joy of fixing things and making your story even better. You'll be happier for it, and your readers will appreciate the extra work.