The Klausenberger Rebbe zt'l lost his wife and eleven children in the Holocaust hy'd, r'l. He said (Tehillim 33:9), ויעמוד צוה הוא ויהי אמר הוא כי, the Ribono Shel Olam Who commanded the tzarah to come, also commanded ויעמוד, that we should pick ourselves up from mourning and move on with life.
It states (Bereishis 27:34), מאוד עד ומרה גדולה צעקה ויצעק, "[Eisav] cried out an exceedingly great and bitter cry." L'havdil, Mordechai HaTzaddik also cried, as it states (Esther 4:1), ומרה גדולה זעקה ויזעק, "He cried a loud and bitter cry." But this time, it doesn't say מאוד עד, that it was an exceedingly bitter cry. Because a Yid knows that everything comes from Hashem, and therefore, nothing is terrible for Klal Yisrael.
Yaakov caused Eisav to cry when he took the brachos. Years later, Haman caused Mordechai to cry with his gezeirah. The Midrash (Esther Rabba 8) says that Yaakov was punished for causing Eisav to cry, and therefore, years later, Eisav's descendant (Haman) caused Yaakov's descendent (Mordechai HaTzaddik) to cry. Nevertheless, Mordechai's crying was less; it wasn't מאוד עד, because a Yid knows that everything is for our good.
It states (Tehillim 119:8), מאוד עד תעזבני אל. Dovid davened that Hashem should never take away his emunah until he feels that the tzaros are מאוד עד, exceedingly great, beyond what he can endure.
I heard from a tzaddik that the earth is round, without corners, so a person should never feel he got to "the end of the world." No matter what trouble he is going through, there is always hope.
During the Apter Rav's zt'l lifetime, there were harsh gezeiros on Klal Yisrael. The Apter Rav said that after his petirah, when he is brought to heaven, he will refuse to enter Gan Eden until the evil decrees are annulled.
The Apter Rav was niftar, but the decrees remained. They even worsened. People asked Reb Yitzchak Meir of Zinikov zt'l, the Apter Rav's son, to explain.
Reb Yitzchak Meir placed his head on the table, and when he raised it, he said, "My father came to me and he explained the pasuk (Tehillim 48:10), היכלך בקרב חסדך אלקים דימינו. This can be translated, אלקים דימינו, we thought it was אלקים, middas hadin, harsh judgment. However, היכלך בקרב, in heaven, people see that it is all חסדך, Hashem's kindness. My father explained that now that he is in heaven, he sees that even the harsh decrees are for our good. So why should he stop the decrees? They are all for our benefit."
The Ramban had a student who had many questions about the way Hashem was leading the world. In his opinion, life should be better for Klal Yisrael.
This talmid became ill and was about to die. Now the student had another question: Why did he deserve to die so young?
The Ramban came to the student and gave him a kameia (amulet) and said, "Hold on to this kameia, and when you die, ask the malachim to bring you to a chamber in heaven called למשפט כסאות, 'Chairs of Judgment.' When you get there, ask Hashem why you had to die so young, and ask Him your questions on the way He runs the world."
The student was niftar, and the Ramban waited to hear what happened.
Once, the Ramban was studying Torah near a window when the window suddenly opened, and the Ramban saw his student. The student said, "After my death, I showed the kameia to the malachim, and I asked to be brought to למשפט כסאות. The malachim took me up to this very high chamber. Nothing could stop me because I held the Ramban's kameia. In this chamber, I was about to ask my questions before Hashem, but suddenly, all my questions disappeared. They didn’t bother me anymore. Because in heaven, one sees that everything is just" (Mishnas Sachir, Avos 6).
The Ramban clearly tells us that crying is permitted. The problem is when it becomes excessive. The Ramban quotes the Gemara (Moed Katan 27:), יותר נפש על להתאבל באסרם מדאי, "one may not mourn the dead too much."
