Perhaps, depending on your background, you're prefer a form of aeternum? And unum is "one" as in the number, and again you may prefer a "one and only" kind of word, like solus. But I don't have a dictionary on me...
This may be the best one online...
"
Forever" on
www.perseus.tufts.edu
Maybe
sola aeterna or the like.
Tell us how it goes...
Colin Jensen, real-live Latin Teacher