Not worthless. But it is inconsistent with the Scriptural basis of the prayer and the tradition of the Church. Why change it?
I think that’s an acceptable reason to alter a prayer for private use: I have a tendency to say ‘amongst women’ (rather than ‘among’ as in most translations) largely because the resultant pronunciation is smoother for me. There’s also a prayer in St Montefort’s rosary devotions, ‘Most Holy Trinity, I offer to thee these three Hail Mary’s’. I find ‘thee these three’ a bit of a tongue twister, so I just alter it a bit to avoid stammering.
This sounds extremely scrupulous. Saying Our Lord vs The Lord is not even a sin, much less a mortal one. It is a private prayer. Sacraments can be invalid, not devotions. Invalid, in Catholic terminology means that no sacrament occured because of a serious defect. However, changing one word in a prayer does not mean that a prayer did not occur. While I would stick with the normal wording of the prayer, I do not think that there is any sin in saying “Our Lord”. Some people say “Blessed art thou among women” vs “Blessed art thou amongst women”. I have always said “amongst”, I realized in the last few years many others were saying “among”. I looked it up and both are used, although most sources now use “among”. My wife was sure that “amongst” was the correct word from having to write the prayer out many times in Catholic school. However, the way it is used most often at my daughter’s Catholic school is “among”.
I am an “amongst” person myself. I didn’t notice for years that many (if not most) other adults were saying “among”. I even looked it up, and most modern versions use “among.”