1. Question: Why is one allowed to say the chapter of tehillim of “Hashem Ro’i” after washing but before saying hamotzi but Rav Moshe Feinstein says one cannot say a “grace before the meal” made up by yourself that asks for success in all endeavors after washing but before eating?
A. The Mishna Berura (166:1:3) quoting a Zohar says that one has a mitzvah everyday to pray for his food before he eats and should do so before washing but if he forgot and first washed then some say that this prayer or paragraph from tehillim can be said before eating but better to say it after say blessing of hamotzi and take a bite of bread.
B. Normally the halacha is one should not talk between washing and eating because that constitutes a distraction.
Answer: A prayer on Hashem giving you food is needed as part of the meal so it might not be considered a distraction but a prayer for one’s own success is extra so it’s a distraction and Rav Moshe says if one always says it, it might even constitute the prohibition of adding to the Torah, bal tosif, because one is adding on something on his own accord which the rabbis did not feel a need to do (Dirshu note 4).