The quote appears in Walter Kauffman's The Portable Nietzsche, and is attributed to a letter to his sister dated March 1885. It is hard to say without the context of the letter who it refers to, if anyone in particular, but it is unlikely to be directed at psychologists. They were not on his mind at the time, in between writing the two parts of Zarathustra. It is possible that the "immodest enough to wish to profit those whose spirit and will are concealed from him" refers to the "mediocre enthusiasts", like his well-intentioned devotee and by then companion of two years Paul Lanzky, who increasingly annoyed him.
Unfortunately, the published collections of Nietzsche's letters, like Levy's, 1921 or Middleton's, 1969 do not contain this letter. Neither does Kauffman's paper on four unpublished letters. The closest we find is a March 31 letter to his mother and sister, where he complains about Lanzky:
"You will notice that I am once more in my more serene state of mind; the main reason for this is the departure of Herr Lanzky. A very estimable man and very devoted to me -- but what do I care for either of these things! For me he means what I call "cloudy weather," "German weather," and the like. Really, there is nobody living about whom I care much; the people I like have been dead for a long long time -- for example, the Abbé Galiani, or Henri Beyle, or Montaigne."
He explained more concerning his ambivalence earlier, in a February 1885 letter:
"Tomorrow Herr Lanzky leaves me, a very decent man who nevertheless impressed on me again the value and necessity of solitude for me. I shall be careful not to lose another winter in this way. To be sure, I have every reason to be very grateful to him for many signs of good will and consideration; but one thing is a hundred times more important to me than anything else."
And he was even more explicit in a December 1884 letter to Overbeck, after Lanzky published a panegyric about him:
"But yesterday when he gave me a long essay about me (printed in a Hungarian journal!), I had no choice but to do what I had done last year with Dr. Paneth, also a great admirer and worshipper: namely, to oblige him not to write about me. I do not have the least wish to see a new kind of Nohl, Pohl and"Kohl" sprout up around me — and prefer my absolute concealment a thousand times to being together with mediocre enthusiasts."
Philippa Foot in Virtues and Vices places this quote into a general context of Nietzsche's disdain for Christian morality, and its charity and pity in particular, but this seems far fetched in a private letter:
"Pity, he says, is a temptation to be resisted at all costs; he thinks of it as a kind of poison to the compassionate man, who becomes infected by the sufferings of others. ‘The suffering of others infects us, pity is an infection’ (WP 368).2 Nor does he believe that pity relieves suffering. Now and then it may do so, but more often the object of our compassion suffers from our intervention in his affairs. He suffers first from the fact that we are helping him. ‘Having seen the sufferer suffer, I was ashamed for the sake of his shame; and when I helped him I transgressed grievously against his pride’ (Z II ‘On the Pitying’). ‘It seems to me that a human being with the very best intentions can do immeasurable harm, if he is immodest enough to wish to profit those whose spirit and will are concealed from him...’ wrote Nietzsche in a letter to his sister in 1885... Nor did Nietzsche think that good motives lay behind most charitable acts. Charitable and helpful people ‘dispose of the needy as of possessions... One finds them jealous if one crosses or anticipates them when they want to help’ (BGE 194)."
