Question: As a staunch listener to Rabbi Mandel’s Shiurim, I heard him say many times that if one has a problematic child, he should say positive things about him. How does this work, and how is this included in Bitachon?
Answer: There are many Seforim that mention this Yesod. I once saw a fascinating Sefer “L'kaf Zechus” from Eretz Yisroel that expounds on this with stories and Chazal’s. R' Tzadok mentions this in Tzidkas HaTzaddik, and so does R' Chatzkel. You should go through the Pele Yoetz where he expounds on this. When you have positive thoughts about someone, you really think that Hashem is going to make the situation more positive according to your thinking. Now THAT’S Bitachon.
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Question: How does one who is yearning to have boys, console himself after having several girls? We find in Chazal (Bava Basra 16b): אוֹי לוֹ לְמִי שֶׁבָּנָיו נְקֵבוֹת, בָּאָה לָעוֹלָם מְרִיבָה. And one of the Amoraim was sad after having a baby girl. While indeed he was consoled by his father, the Gemara says that his consolation was not substantial.
Answer: You have to realize that there are many Chazal’s for many situations and different types of people. And you have to use your Sechel to know when to apply them. For example, if a person reads up on all the Chazal's about Gehinom and then gets depressed, that means that those Chazal’s were not meant for him. Or if somebody learns all the Chazal's that discuss only the topics that are not Nogea L'maase, that’s also not good.
You have to know that every situation has a Ma'ala. I myself have six daughters and one son. I know someone from a certain family who had only girls, and he excelled in certain areas that the rest of his family did not, and all his sons-in-law got great jobs. YOU HAVE TO FOCUS ON THE GOOD IN YOUR MATZAV.