(א) כי בשש משה... בְּי"ו בָּא שָׂטָן וְעִרְבֵּב אֶת הָעוֹלָם, וְהֶרְאָה דְּמוּת חֹשֶׁךְ וַאֲפֵלָה וְעִרְבּוּבְיָה, לוֹמַר וַדַּאי מֵת מֹשֶׁה לְכָךְ בָּא עִרְבּוּבְיָא לָעוֹלָם, אָמַר לָהֶם מֵת מֹשֶׁה, שֶׁכְּבָר בָּאוּ שֵׁשׁ שָׁעוֹת וְלֹא בָּא וְכוּ' כִּדְאִיתָא בְמַסֶּכֶת שַׁבָּת (דף פ"ט)
(1) כי בשש משה [AND WHEN THE PEOPLE SAW] THAT MOSES DELAYED LONG — ...On the sixteenth of Tammuz Satan came and threw the world into confusion, giving it the appearance of darkness, gloom and disorder that people should say: “Surely Moses is dead, and that is why confusion has come into the world!” He said to them, “Yes, Moses is dead, for six hours (noon) has already come (בשש = בא שש) and he has not returned etc.” — Shabbat 89a (cf. Rashi and Tosafot there and Tosafot on Bava Kamma 82a ד"ה כדי).
אל תצר צרת מחר כי לא תדע מה ילד יום שמא מחר בא ואיננו נמצא מצטער על העולם שאין שלו מנע רבים מתוך ביתך ולא הכל תביא ביתך רבים יהיו דורשי שלומך
The Gemara quotes additional statements from the book of Ben Sira: Do not suffer from tomorrow’s trouble, that is, do not worry about problems that might arise in the future, as you do not know what a day will bring. Perhaps when tomorrow comes, the individual who was so worried will not be among the living, and he was consequently upset over a world that is not his. Prevent a crowd from inside your house, do not let many people enter, and do not even bring all your friends into your house. Make sure, however, that a crowd seeks your welfare, and that you have many allies.
Implication- the value of anxiety can be found in its ability to prepare you for the eventual "Vaay", the actual negative thing you were worried about. Worry allows you to minimize the discrepancy between before and after the traumatic, anxiety inducing event.
Someone who is prepared for something not good is called a "worrier". And since Leah was expected to marry Esav she was a worrier... Therefore she was always praying for a better situation...
May it be Your will, O Lord my God,
that this enterprise be for healing and that You should heal me.
As You are a faithful God of healing and Your healing is truth.
Because it is not the way of people to heal, but they have become accustomed.
Rav Aḥa is saying that people should not practice medicine as they lack the ability to heal; rather, healing should be left to God. Abaye responded and said: One should not say this, as it was taught in the school of Rabbi Yishmael that from the verse, “And shall cause him to be thoroughly healed” (Exodus 21:19), from here we derive that permission is granted to a doctor to heal. The practice of medicine is in accordance with the will of God.
וַיֶּחֱלֶ֣א אָסָ֡א בִּשְׁנַת֩ שְׁלוֹשִׁ֨ים וָתֵ֤שַׁע לְמַלְכוּתוֹ֙ בְּרַגְלָ֔יו עַד־לְמַ֖עְלָה חָלְי֑וֹ וְגַם־בְּחָלְיוֹ֙ לֹא־דָרַ֣שׁ אֶת־יְהוָ֔ה כִּ֖י בָּרֹפְאִֽים׃ וַ
In the thirty-ninth year of his reign, Asa suffered from an acute foot ailment; but ill as he was, he still did not turn to the LORD but to physicians.