Sartre is making a similar point in the course his discussion of 'The Origin of Nothingness'. He is out to show that between our actions and the motives we have for those actions there is inevitably a gap: our motives themselves are ineffective to bring about our actions. It's always possible that we act differently, or don't act at all. And though I may have a motive to act in a certain way, nothing in that motive is sufficient to make me act that way (rather than act differently, or not act at all). Sartre speaks here of "that nothing which insinuates itself between motives and act". (71) For him, this idea has to carry some theoretical burden: because of it we can experience human freedom: "the structure of motives as ineffective is the condition of my freedom" (ibd.). If we couldn't perceive our motives as in the end always ineffective, we wouldn't be able to see ourselves as free. But since we do experience them as ineffective, we are aware that we have to make up our attitudes and actions over and over again; thus "freedom [...] is characterized by a constantly renewed obligation to remake the Self which designates the free being". (72)
And he gives a very helpful example for this line of thought: the gambler. Sartre invites us to consider the case of a man who has been a habitual gambler and now wants to stop it—he's "freely and sincerely decided not to gamble any more." (69) He might have all sorts of good reasons (perhaps he realizes he risks financial ruin, or he feels he is a disappointment to his family and wants to correct that). So he makes a resolution of 'not playing any more'. Now, having thought it through and being determined to stick to the resolution, that should be enough to make his future actions comply with it, shouldn't it? However, what he actually experiences shows that this was an illusion: "when he approaches the gambling table, [he] suddenly sees all his resolutions melt away." (Ibd.) What he realizes is that however determined his stance might have been, it's in the past, and if he wants his reasons to guide his actions, he has to re-make the decision, he has to get to the resolve again:
I should have liked so much not to gamble anymore; yesterday I even had a synthetic apprehension of the situation (threatening ruin, disappointment of relatives) as forbidding me to play. It seemed to me that I had established a real barrier between gambling and myself, and now I suddenly perceive that my former understanding of the situation is no more than a memory of an idea, a memory of a feeling. In order for it to come to my aid once more, I have to remake it ex nihilo and freely. (70)One thing we should note here is that, in this example, freedom is not exactly welcome. On the contrary, it is a thoroughly disturbing insight that at this point, the gambler is free, i.e. exactly not bound by the resolution he had made in the past. Or, to put it the other way round: there was a certain comfort and security in the idea that a resolution might be enough to control one's further actions; the insight that it doesn't, that in the event there is nothing to stop one from acting against a resolution, destroys that comfort and security. Being free isn't always easy. (More often than not, according to Sartre, we attempt to look away from our freedom in an attitude which he calls 'Bad Faith' and illustrates most ingeniously a few sections after the passage I've just quoted.)
That we have to "remake the Self" (72) in this way is because the past that is involved here is a form of unreality (and not reality). (Notice, in the quote that I'm about to give, how Sartre tellingly puts the word 'magic' in the mouth of his gambler.) Or again, as Sartre puts the point:
The not gambling is only one of my possibilities, as the fact of gambling is one of them, neither more nor less. I must rediscover the fear of financial ruin or of disappointing my family, etc., I must re-create it as experienced fear. [...] After having patiently built up barriers and walls, after enclosing myself in the magic circle of a resolution, I perceive with anguish that nothing prevents me from gambling. (72)And there we have it, the "nothing which insinuates itself between motives and act". (71) It's a nothingness (as Sartre calls it) which in this case comes from the past, a form of unreality, being involved here. (Other forms of unreality might come into play, but in this example, just as in my own example in my post about Kierkegaard's definition of self, it happens to be the past.) According to him (if I understand correctly), such nothingness is continuously involved in the relation of a human being with itself, and thus an inevitable component of it. When we become aware of this, one effect is that we become aware of our own essential freedom (which we experience in what Sartre calls 'anguish'). What's perhaps even more interesting is that, in continuously doing so, human beings (beings with consciousness and a self) perpetually bring nothingness into the world—that's where it actually comes from.
(But this is a topic for another time.)
 In chapter 1 of his Being and Nothingness. Translated and with an introduction by Hazel Barnes. NY: Washington Square Press 1992. Quoted with page numbers in the text.