I’m currently writing a murder mystery novel in where the MC has to solve a murder that happened in 1998. In the climax she is fighting off the killer but then her love interest comes in and kills the guy, and I want to make her extremely upset but I also want her to seem emotionless like she’s hiding her true feelings because that’s something she does a lot in the book. The thing is, she’s already watched people die, her parents, so it’s not like it’s new to her but it’s still traumatic. She never cried much after she lost her parents because then it would seem too real, so this is also around the point where she’s accepting her parents’ death.
Having your cake and eating it
Many questions on this exchange take the form of "How do I do x without doing x?"
There is no answer to these questions without some kind of frame challenge to the original idea. Clearly you can't be 'extremely upset' and 'emotionless' at the same time, any more than you can eat all your cake and still have cake.
The way 'cake' has been handled for centuries is to feature the cake, display it prominently for everyone to admire, and build anticipation around it. Cake is a beautiful, but temporary confection. Despite the weird recent internet trend in cakes that look like mundane objects, it isn't a cake until the knife cuts a slice.
Your story is a cake
If you can stand the metaphor: your character's emotionless facade is a cake that has been displayed to readers for the bulk of the novel. They know that cake is temporary, it cannot last – no matter how elaborate and beautiful the icing.
This cake isn't some bakery window display gathering dust, it's THE cake baked specially for THIS story –– you've invited your readers to the banquet and they've been looking at this decorated cake the whole meal. At the climax, you will cut the cake and reveal what's inside.
No one will be impressed if your cake turns out to be an icing-decorated cardboard box. Nor will they feel satisfied if you leave the cake untouched on the shelf and tell everyone to go home.
Building anticipation makes cutting the cake an important event. It feels like the climax of the party. Everyone watches in excitement, that first slice is a point of no return. After the first cut, it's no longer an object on display. The cake is divided and shared, enjoyed by your readers.
Stories need to evolve
I want to make her extremely upset but I also want her to seem emotionless like she’s hiding her true feelings because that’s something she does a lot in the book. Because the thing is she’s already watched people die, her parents, so it’s not like it’s new to her but it’s still traumatic. She never cried much after she lost her parents because then it would seem too real, so this is also around the point where she’s excepting her parents’ death.
It sounds like this would be THE moment to put a knife in that flawless facade and reveal the cake inside.
Stories (and most protagonists) go through a change. By setting up a character who is not dealing with her past trauma, you've created a promise to the reader that this will have some pay-off at the climax.
Your character's 'emotionless' facade is a cake you've put on display this entire banquet. It is fragile and beautiful – other characters have commented on it, it draws attention to itself through her behavior. The reader knows this 'cake' can not last forever sitting on a shelf, likewise this character can't go on forever suppressing her emotions. She would become a broken character – an emotionally frozen villain like Miss Havisham from Great Expectations (with a literal untouched 'cake' sitting on display covered in dust).
Other writing advice applies here: kill your darlings, and force the protagonist endure cruel pain and conflict – agony and protagonist have the same root word.
You don't need a degree in psychology, you just need to understand that as author you are hosting a party – if you've been teasing us with cake the entire story then you must get the knife and cut that preciously constructed cake into slices to serve to your guests. Readers will be satisfied, but perhaps more importantly they will feel like the story has reached it's conclusion.
If this is a conclusion to your story, you might want to look into some "Thousand Yard Stares" and similar reactions... essentially when a character's andrenelin and emotional drive run out. They are responsive but look like they are going through the motions... they aren't emotionally invested in anything. One of the better and more frequent uses I've seen was in Buffy the Vampire Slayer. In the finale of Season 2, when Buffy has to kill Angel moments after his soul is returned to him to save the world (long story, she has a quiet emotional breakdown. Buffy leaves Sunnydale when we next meet up with her, she's withdrawn from all her social connections and support networks, all of which had been up ended. In season 3 premier, we find her using a false identity to hide from the world.
Another example is Season 5's "The Body" where Buffy returns home to find that her mother has passed away suddenly (in what is one of the few deaths not related to the supernatural in the series)... the episode is shot with an entirely diegetic soundtrack (no soundtrack exists that doesn't have a source internal to the story... there is sad music played over a speech or horrific scare cord when a vampire appears) that highlighted the isolation of the moments of revelation.