En su vida se había sentido más nerviosa
 

En su vida se había sentido más nerviosa

It seems to me something like.
She never felt so nervous in her life.
But there is no negative word, so is that a reasonable interpretation.

Yes. When this expression (and its variations with other possessive pronouns) comes before the verb in a clause, and especially when it starts the clause, it always negates the verb and it always means "never" or "never in his/her(your, my, etc.) life", unless the context of the clause makes that interpretation unlikely or impossible.

When it follows the verb, it is not automatically negative, and it has the natural meaning "in his/her(your, my, etc.) life".

Thank you interesting