RobH_en_tn/2jn/01/05.md

you, lady ... writing to you

These instances of "you" are singular. (See: rc://en/ta/man/translate/figs-you)

not as though I were writing to you a new commandment

"not as though I were commanding you to do something new"

but one that we have had from the beginning

Here, "beginning" refers to "when we first believed." Alternate translation: "but I am writing to you what Christ commanded us to do when we first believed. (See: rc://en/ta/man/translate/figs-explicit)

beginning—that we should love one another

This can be translated as a new sentence. Alternate translation: "beginning. He commanded that we should love one another"