Roald Dahl says it's OK.

...some people, when they have taken too much and have been driven beyond the point of endurance, simply crumple and give up. There are others, though they are not many, who will for some reason always be unconquerable. You meet them in time of war and also in time of peace. They have an indomitable spirit and nothing, neither pain nor torture nor threat of death, will cause them to give up.

- Roald Dahl, THE SWAN

When you’re writing, it is rather like going on a very long walk towards many crossroad, valleys, mountains and things. And you get the first view of what you see and you write it down. Then you walk a bit further, maybe up on to the top of a hill and you see something else. Then, you write that and you go on like that, day after day, getting different views of the same landscape. The highest mountain on the walk is obviously the end of the book because it’s got to be the best view of all, when everything comes together and you can look back and see everything you’ve done all ties up. But it’s a very, very long slow process. It starts always with a tiny little seed of an idea, a little good germ and that even doesn’t come very easily. You can be mooching around for a year or so before you get a good one. When I do get a good one, I quickly write it down so that I don’t forget it, I don’t dash up here and start to write it. I’m very careful. I walk around it and look at it and sniff it and then see if i think it will go. Because once you start, you’re embarked on a year’s work and so it’s a big decision.

One of the vital things for a writer who’s writing a book or scripts or whatever, which is a lengthy project and is going to take about a year is actually how to keep the momentum going. To be confronted with a blank page is not very nice.

A writer once taught me the finest trick when doing a long book, which is simply said in his own words - “ When you are going good, stop writing ”. And that means that if everything’s going well and you know exactly where the end of the chapter’s going to go and you know just what the people are going to do, you don’t go on writing and writing until you come to the end of it because when you do, then you say, " well, where am I going to go next? And I get up and I walk away and I don’t want to come back because I don’t know where I want to go ".

But if I stop when we’re going good, as the writer said, then we know what I am going to say next. You make yourself stop, put my pencil down and let go of the keyboard and everything. You walk away. And we can’t wait to get back because I know what I want to say next and that is incredible and we have to try and do that.

Every time, every day all the way through the year or so. If we stop when I am stuck, then we are in trouble.