Musar for Menachot 219:17

Nowadays, when we do not have an altar or priest to perform all these duties on our behalf, the only way we can compensate for this lack is by studying the relevant sections of the Torah with a grieving heart and confessing our sins, so that at least the verse in Psalms 51,17: זבחי אלוקים רוח נשברה, "True sacrifice to G–d is a contrite spirit" can be applied to us. Our sages are on record that he who occupies himself with the portions of the Torah which deal with the sacrifices by trying to understand their deeper significance to the extent that he imposes fast-days upon himself is considered as if he himself had become a sacrifice of sweet smelling fragrance. The author of Tolaat Yaakov dwells on this on page 67 of his book. He writes as follows: "The mystical dimension of fasting is that it is viewed like an altar, ready to atone for the sins of the person fasting. By reducing the intake of food and drink one reduces one's body fat, sacrificing it to G–d combined with one's prayers. Towards evening, when such a person feels increasingly weak, he is as though the fire which is one of the basic elements man is made of becomes predominant within him so that it symbolizes the burning wood-pile on the altar of which the Torah said that its fire should never be allowed to be extinguished. This body-heat then consumes part of the fat and blood the body has accumulated. This physical process parallels what used to happen on the altar when the animals were burned. On a still more fundamental level: just a there are איברים, "limbs," i.e. individual parts of sacrifices in the Celestial Regions whose function it is to be consumed by Heavenly fire, so there are corresponding איברים in our world whose function it is to be consumed by the fire on our terrestrial altar. Just as the smoke from the altar rises heavenwards, so the spirit of man leaves his body and rises turning into ריח ניחוח, a sweet smelling fragrance for G–d. All of this takes place on the day the person who is engrossed in the study of the deeper meaning of the sacrifices observes a fast. A person must concentrate on this objective wholeheartedly with both body and soul. He must mentally pepare himself for this as if he were preparing his sacrifice for the altar. When he does this he will obtain forgiveness for the transgressions he has been guilty of. In the above mentioned manner man can transform himself into a fully fledged altar. He can "offer" all the different kinds of sacrifices on this altar.