Rabbi Chaim Luzzato, one of the main students of the Arizal, wrote several books on the teachings of his rabbi. Some of the books that he wrote are Etz Chaim and the Eight Gates, one of the Eight Gates is Shaar Hakavanot (The gates of intentions) that deals with the proper kavanot (meditations) that a person needs to have during the events of jewish life.

In Shaar Hakavanot, the Ari explains that from 17 of Tammuz to 9 of Av we have 21 days and that it is a very good custom for any one that is Baal nefesh (elevated person) to sit mourning on the destruction of the Beis Hamikdash. The mourning should be done after midday because we have a concept that after midday there  is an awakening of the judgements and it goes also as it happened, the beginning of the burning of the Beis Hamikdash was after midday. Someone that is doing this custom is getting a big benefit to his being. Also the person has to continue (not to do only during these three weeks) making tikun chatzot at midnight.

If you feel that it is already too much, all the restrictions that we have according to the halacha plus doing kabbalistic stuff on top of that! Don’t feel bad, rabbi Shephard explains that there are people that their soul is connected to the destruction of the Beis Hamikdash, these people are susceptible not to feel good after midday...