The Chasam Sofer zy”a (Drashon Chasam Sofer, page 224) also writes that the words “Hashem sefasai tiftach” are a tefillah we say before Shemonah Esrei in which we ask Hashem to help us daven correctly. He explains the need for this by saying that before one speaks to the King of Kings, he may become frightened and confused, and thus unable to articulate himself. He may get so scared that he cannot even speak. Therefore, we ask Hashem to open our lips so that we should be able to express ourselves, despite the great awe and fright that we feel.
He quotes the Haflaah zy”a as saying that this is the meaning of the word “laymor”. The pasuk is saying that Moshe asked Hashem to be able to say his request, despite the great fear he felt.
The Chasam Sofer goes on to ask why we only find Moshe needing to make such a request this one time. He answers by saying that Moshe was not davening to be able to enter Eretz Yisroel, which possesses a kedushah beyond imagination. Moshe felt that this holiness even went beyond the kedushah he felt when he ascended to Shomayim for 40 days, as the land of Eretz Yisroel is even holier than the Shomayim of Chutz La’aretz. Therefore, he felt like he unable to even open his lips to utter his tefillah and he needed to ask for Hashem’s help.
