Whenever dealing with kirlia who wish but are unable to evolve into gallade (or a pokémon that’s unable to evolve into one of its evolutionary branches, such as eevee or snorunt), it’s extremely important to first emphasize both their own self-worth and the worth of their alternative evolution, gardevoir. Teach them that they can be strong, no matter what evolutionary form they take, and inform them of the strength of their alternate evolutions. In gardevoir’s case, this may be as easy as showing them videos of Champion Diantha’s gardevoir, as she’s well-known (to the point of being an idol for quite a few kirlia I know) for her strength and grace.
Additionally, consider teaching them techniques that can be translated into the evolutionary form they can take. Consider teaching your kirlia Focus Blast shortly after they evolve, or teach them to move about the field using the fencing techniques they would utilize as a gallade. This may help your kirlia feel as if they can get the best of both worlds, so to speak.
Above all else, though, reinforce the idea that you value your pokémon, no matter what forms they take. Let them know that they are still vital parts of your team, and show them—through affection or through displays of your trust—that you will always consider them to be your close companions, even if they can’t evolve into the form they wish.
Also, regarding the kirlia who may be upset that she doesn’t have a choice in the matter, the above advice is only if you’re sure that’s what she’s concerned about. I highly recommend that you communicate with your kirlia first, as what’s bothering her may have nothing to do with evolution at all.
Best of luck to the both of you!
The Pokédex According to Bill 