Likutei Halakhot, Orach Chaim, Laws of Phylacteries 5:8/ליקוטי הלכות·אורח חיים·הלכות תפילין ה׳:ח׳

1And this is the aspect of “Long-Tempered (Erekh Apayim) for tzaddikim and for the wicked” (BK 50), as tzaddikim need this trait of Erekh Apayim, which is the aspect of holy elderliness, the aspect of Arikhath Yamim weShanim/Long Days and Years, the aspect of Chayim Arukhim/Long Life, mentioned above. That is, to not fall in the oldness of the Sitra Achra, God forbid, that his service should not old and weak for him, but just “renew his youth like an eagle” all the time, and every day begin new with additional sanctity and service, and never get tired or weary, and not be confused by anything, which this is the aspect of Erekh Apayim. For, sometimes one who serves Hashem gets weary on account of having suffered so many trials etc., and thereby he could fall into oldness of the Sitra Achra and exhaustion, God forbid. Therefore he needs to strengthen himself a great deal with the trait of Erekh Apayim, all his days, extending his temper and spirit for all the toils and burdens, beginning new every time. And this is the aspect of Erekh Apayim for tzaddikim. And likewise there is the aspect of Erekh Apayim for the wicked, as Hashem Yithbarakh also extends his temper for them all the days of their lives, in order that they return, as written, “Until the day he dies You wait for him; if he turns back etc. [You receive him immediately].” Therefore the wicked themselves also have to bolster themselves in this trait of Erekh Apayim, that their wickedness should not confuse them from returning to Hashem Yithbarakh, and that their hearts should not falter nor their minds be downcast from the amount of their transgressions, especially if they have already began a few times and fallen from it etc. Nevertheless they should be confident in His great mercy and prolong their patience and spirit over all that happens to them, and strengthen themselves each time to begin anew. Perhaps he will attain from now on to have compassion on himself, to leave his previous way and thoughts, and never ever be old and weak in his eyes. For the essence of return lies in the aspect of renewing one's life whenever it is necessary to renew his days that have passed in darkness, in the aspect of (Lam. 5), “Hashiveinu H' eleikha wenashuvah chadesh yameynu keqedem/Return us, Hashem, to you, that we may be restored; renew our days as before.” Hence everyone needs the aspect of this trait of Erekh Apayim. And this is, “Long-Tempered (Erekh Apayim) for tzaddikim and for the wicked.” And the rest of the levels of other people in the world are are included in them, for the whole keeping of their service and vitality is by this trait of Erekh Apayim, as mentioned.