The Power of the 3 Weeks
The Zohar (in Chelek Beis, pg 78b) writes that Yakov Avinu took the months of Nissan and Iyar for himself (hence Yetziyas Mitzrayim, Kabbalas Hatorah), and Eisav took the months of Tamuz and Av for himself (and hence, the 17th of Tamuz and Tisha Be'av), but only the first 9 days of Av belong to him.
It says in the Mishna that they stopped gathering wood for the Mizbe'ach from the 15th of Av and on, because the strength of the sun finally breaks at that point, and the wood for the Mizbe'ach must be gathered only while the sun is at its strongest. (Once the sun is no longer as strong, the wood can be slightly damp inside and it won't burn as well.)
The Jewish people are compared to the moon, which renews itself each month. When Moshiach comes, the moon will be as bright as the sun. Esav chose this world over the next world, and the ruler of the heavens in this world is the sun. When the sun is the strongest, the koach of Esav is the strongest. But only until Moshiach comes...
The Beis Ahron of Karlin at the end of Parshas Pinchas brings a Medrash that says that the months of Tamuz, Av and Elul have no Regalim in them. That is why Hashem gave us three Regalim in the month of Tishrei, as if to "pay us back". The Beis Ahron writes that the 3 weeks are a very high time. These 3 weeks symbolize the 3 upper worlds (Ga"r or Gimmle Rishonos; Keser, chachma and Bina), and according to Kabbala, these three worlds are above time and space. That is why, says the Beis Ahron, the 3 weeks are a time when Hashem is so "hidden". Not because it is a "dark" time, but rather because the world cannot be "Sovel" (tolerate) the great light of this period of the year. But when Moshiach comes, he writes, these three weeks will be revealed and they will become great Yomim Tovim. And the Beis Ahron ends by saying that that is why we usually read Parshas Pinchas during the 3 weeks, because Pinchas has in it all the Regalim...
If you are finding the struggle very difficult now, it is understandable. The Koach of Esav is at its height during the three weeks. From Rosh Chodesh Tamuz until the 9th of Av is 39 days. We are now in middle of those 40 days. At the end of these 40 days, the sun's strength starts to wane. That means that in middle of the 40 days, the sun is at its peak. Now is the most hidden time. The world cannot be "sovel" the great light, and when the keilim are not able to hold the light, they break. That is why the walls of Yerushalayim were breached and the Beis Hamikdash was destroyed. The "Keilim" of the yidden were not worthy of maintaining the great light, so when the light became the strongest, it broke the vessels. And that is perhaps why so many of us are struggling now more than any other time of the year.
But Rabboisai, the wood for the Holy Mizbe'ach can only be gathered NOW when the sun is at its PEAK. This is perhaps because the greatest sacrifices for Hashem are done in the most hidden times. Davka now, the light of the Ge'ulah begins to shine forth.