So my agent was sitting on a delayed bus into work, and I was walking along a long and snowy road in lieu of spending half an hour digging my car out, and we were on the phone discussing the latest version of my new novel. Basically she loves it, and thinks it's very nearly ready to fly: she awarded the ending three hankies and we've settled on a great title. She even spontaneously suggested something for the ending which I'd wanted to do all along but hadn't dared. Her only reservations were about some of the new material. I've totally re-written the first chapter and written an epilogue from scratch, so these are things which have had enough passes to be okay of themselves and to apparently do the job that needed doing. But when she pointed out the ways in which they're not really working, I felt, "Doh! Of course!"
Make no mistake, these aren't technical or high-level slips: they're common mistakes in storytelling that I see all the time in the work of aspiring writers. You'd think I would have pounced on them in my own, or even not committed them in the first place. But, since I was feeling cheerful by then, instead of beating myself up I started wondering why I hadn't seen them. And the clue, I think, is in "common mistakes"; these are things which are a very natural outcome of how most of us set about writing a story.
For example, at the macro level I see many novels where the real story doesn't start till Chapter 4: Chs 1-3 are all just trundling towards the starting grid. And then there's the cry that your research is showing; historical or geographical facts are deliciously seductive to the writer, and it's by making the detail believable that we get the reader to suspend their disbelief in the story. But we're storytelling, not writing a gazeteer or a history book, and anything that doesn't serve the story is weakening it.
And at the micro level I'm sure it's why I spend so much time saying, "Don't tell and then show". Don't tell us the picnic was a disaster then write the scene: one or the other ("tell" is an important part of any writer's toolkit) but not both. But this too, arises not because the writer is careless or talentless, but because of how stories come to us. Either explicitly or intuitively the writer realises that for the purposes of character-in-action and plot the picnic will be a disaster. So they write that fact down, then set about making it happen.
So don't beat yourself up. Ch1-3 are all backstory that you needed to work out: they're process writing, not a failure or mistake, and now they've achieved their purpose and can be cut. And it was only by putting all the researched material in, that you discovered which bits become part of the story-stew and which stay floating factily on the top. And think about it: maybe the writing down of that initial "tell" about the picnic helped as a note to yourself might, getting your creative brain to focus on the picnic's main purpose in the story, and not get bogged down in what's in the hamper.
Instead of regarding such things as mistakes, maybe we should see them as a dressmaker's tacking stitches; the wooden scaffolding over which the arches of the bridge are formed; the paper collar round the soufflé dish that you take off once the gelatine has set. They're part of the fundamental nature and process of creating that thing. But since they are functional they can be hard to spot, and it might be canny to do a pass through the novel purely looking for them. But even as you do, don't scold yourself for putting them there: without them, the story might never have got built at all.