For as long as Riza could remember, she's had a wall. This wall, existing only inside her and her mind, has served as a way to keep two important things separate: her emotions and her actions. She couldn't allow herself to get emotional over anything she did (although even that rule would sometimes fail to be followed, for sometimes this wall couldn't stop a flood of true emotion once Riza was alone) and she certainly couldn't let her emotions get the best of her and dictate her actions (that was a rule that had to be followed; too many people depended on her for her to let emotions cloud her judgement). She thought this wall was strong. She thought it would never collapse.
That was before she learned how weak her wall could really be.
That was before she was told that Roy Mustang was dead.
At first, Riza didn't- couldn't- believe it. The defense mechanism commonly known as "denial" was working in overdrive, trying its best to make the undertone of Lust's words sound impossible, trying its best to convince Riza that she was lying.
It didn't work. In that moment, Riza couldn't count on denial. Maybe she had been able to in her adolescence, trying to deny her real feelings for her father's student and trying to deny that she really was killing innocent people in Ishval (the latter was a short kind of denial, because being a killer is something that only the truly delusional can deny), but she couldn't now.
To put it bluntly, denial was useless to her now.
So, it's really no wonder that the realization of this, coupled with the sinister smile of the Homunculi and the realization of its connotation hitting her square in the gut, caused Riza's wall to collapse. When that happened, she wasn't exactly herself anymore. She had become her emotions, those intangible things somehow controlling her actions.
It was her anger that repeatedly pulled the trigger, causing bullet after bullet to pierce the skin of the wicked monster that had done this, that had taken Roy and his vision from the world and caused Riza's senses to collapse with her wall. It was her anger that pushed the scream out of her mouth, loud and primal, desperate and so utterly furious that Riza was surprised she wasn't seeing red. It was her sorrow that brought her to her knees, tears streaming down her face as she finally was allowed to grieve, knowing in the back of her mind that she would soon meet the same fate as the man she was supposed to protect. It was that overwhelming sense of hopelessness that made her give up, that would have been the ultimate reason for her death...but then, a miracle happened. Roy appeared, not as a memory or a specter but as a person, a still living, still breathing person. That was when she pulled herself together and became herself again. That's when her wall started rebuilding.
Anytime she thinks about it now, it scares her. That wall, for all intents and purposes, was the thing keeping her sane. It was the only thing keeping her from being consumed by what would be, in some situations, violent emotions...and it had fallen as easily as a house of cards falls in a strong wind. It had collapsed with pure ease because she'd been told Roy Mustang was dead.
The way Riza figured, this meant one of two things: either her wall wasn't as strong as she thought or, when it came to the colonel, there's a part of her that would always be weak.
She had to hope that the latter wasn't true.
Riza knows she can't afford to experience another collapse.