I started posting about the editing process a couple of weeks ago, and I mentioned then that I had a whole series about it. This is the second post in that series.
When you first write a story, you've just poured hours and days and weeks and months into crafting that tale, and getting it right. Even if you know, objectively, that some of the scenes were slapdash, and you left marks within the text to denote where you should expand or truncate or otherwise fix, you're going to feel like you're killing your own flesh and blood if you change any of the words.
As you gain distance from those words, though, the more objective you are about them. If you leave them be for a couple of weeks, you may find places where the writing is uneven, or some of the word choices were wrong. Leave the draft for a few months, though, and, if you're anything like me, you'll be wondering who the heck wrote it when you next pick it up.
Better writers than me have advised putting the manuscript into a drawer, or a trunk, or otherwise putting it away where you can't see it. Nowadays, though, few of us print out our first drafts. We're talking about electronic copy. What I would advise would be to put it in a folder you don't look at every single day, and mark on your calendar when it's safe to look at.
Then, go work on something else. Forget about your first draft for a while. You'll need to do some trial and error before you'll know exactly how long you need, but I started with six months, the first time I took this advice. My trial and error yielded a minimum of two months. There is no maximum; I've picked up things I wrote a decade ago and had no idea what stranger was writing such terrible stories in my voice.
Generally speaking, though, by the time I've finished whatever I picked up to make me forget my last project, I'm ready to face that last project and can handle editing it. If I stop my "distraction" project too soon, though, and pick up the first one too soon, I'll be too attached to what I find, and I won't want to edit or cut anything. Once I've talked myself into letting something stay, it stays, often to the detriment of the manuscript.
You should never bank all of your writing success on one story, and you should never work on one thing to the exclusion of all else. That you'll need time to let go of your first-draft words is part of the reason why.
So, when you think you're ready to edit a first draft, make sure you've given it enough space that you'll be able to.