meaning-in-context
In Life (2017) movie, the ISS Crew sends Katrina into space to fix the communication to get back earth online. So the crew reminds her these lines:
David: What is it you say? "Slow is fast", right?
Katrina: Yeah. Slow to go fast.
David: Yeah. Well, that's out the window.
It means you get tired of it pretty fast. It seems "old" after a few days, because the time drags and seems to last longer and longer. And you wonder how much longer it can last, and still there's more, and more, and more.
(After living through winters in Chicago and Massachusetts, believe me, I can tell you it does get old. See the picture below for an example. And there were another six or eight weeks of winter left! )
Yes, this definition is correct for the context. As you state, the definition is
To maintain composure; to avoid an overly emotional reaction; to suppress an instinct or urge.
Later on in the text, it states that she "tries to slow her breathing, the way she's been taught. To stop the images from coming back, from taking over". Also, earlier, there is background given that Riley has been deployed to Afghanistan.
She reminds herself she's not in Afghanistan anymore. She's home, safe, in New York state.
Based on this context, the author is trying to convey the fact that Riley has Post Traumatic Stress Disorder. Some symptoms of PTSD include extreme emotional reactions and intrusive memories of the event.
When the author mentions the "images", this is most likely referring to the intrusive memories, or the things Riley has seen that has distressed her in some way while she was deployed in Afghanistan.
Finally, to tie this point to your question about the definition of "keep it together", the second symptom I mentioned is extreme emotional reactions. Look at the definition of "keep it together" again.
To maintain composure; to avoid an overly emotional reaction; to suppress an instinct or urge.
Therefore, when Riley is "keeping herself together", she is consciously making an effort to not break down because of her PTSD.
Best Answer
It's a translation of an old Latin tag, festina lente, "hurry slowly". It means that trying to do things too fast often means you waste time going back to correct your mistakes: the fastest way of accomplishing something is to work carefully and methodically.