Don't keep secrets for the sake of keeping secrets.
Honestly, in other works, the MC hides sh*t because it could endanger his family and they'd probably not understand it...
In this case, you can tell your fiancee everything (but the transmigration).
Let her know you awakened a unique skill and tell her you'd protect her....
Of course, I get that it's just the author half-assedly copying the other stories and their tropes... but the unique circumstances in this story actually make that duplication just not work....