Hesach Hadaas from Eating

If a person has been eating and resolves [in their heart], gamar belibo, not to eat or drink any more, it is considered hesach ha-daas (a mental interruption in the meal), and the halachah is that if they change their mind and decide to eat or drink further, they have to make a new brachah. However, if this happens during a seudah, a fixed meal (usually after washing for bread), the halachah is somewhat different: Since during a meal a person often decides to stop eating and then reconsiders, the hesach ha-daas isn not absolute, and according to one opinion, there is no need to make a new brachah. In practice, if a person decides not to eat any more during a seudah, since there is a disagreement in halachah whether to make a new brachah, it is best to refrain from eating further. One should not make a new brachah, since it may be a brachah levatalah, a brachah in vain, which is a serious prohibition. After benching one may eat and drink further, making a new brachah. In the case where there is no additional food readily available in their possession, all agree that they have to make a new brachah, as in this case the hesach ha-daas is absolute. Even during a seudah, if one decides not to drink more, and then changes their mind ─all poskim agree that a new brachah is required. Similarly, if he has taken a cup of wine for bentching and has said, “Let us bentch,” or has washed his hands in preparation for bentching, mayim acharonim─he has to make a new brachah. (In that case, he would need to wash mayim acharonim again before bentching.)

https://halacha2go.com?number=181

Practical Halacha: One minute a day. By Horav Yosef Yeshaya Braun, shlita, Mara D'asra and member of the Badatz of Crown Heights.