To be fair to Yvonne with everyone mentioning how long it took her, she did almost unwillingly murder Elsa a few times. If you really think about it, the stabbing incident on the beach is incredibly traumatic. Her hesitancy makes a lot of sense.
Yeah, there's a reason why I never considered Yvonne to be particularly dense. Sure she has her moments, like her complete inability to recognize Elsa was describing her in what she wants in a future suiter, but overall her main holdup is her inability to acknowledge her feelings for Elsa. Anytime Elsa said she was in love with her or did something that made her affection undeniable, Yvonne would immediately justify it away and forget that it happened. Any time she started to feel romantically attracted to Elsa, she'd bottle up those emotions and deny their existence.
The stabbing incident is what really traumatized her though. Prior to that, while she was still denying her feelings such denials were gradually becoming weaker and weaker over time. However, the stabbing incident and the subsequent realization that the Villanous Systems would just keep escalating until it forces her to kill Elsa absolutely terrified her. Her mind mentally paired accepting those feelings with Elsa getting hurt So she doubled down on the denials.
Elsa had to be kept safe -- because Yvonne genuinely loves her and can not bear the thought of her being hurt -- and as long as Elsa is near her, she's in danger. Nothing else mattered, no matter what Elsa said or Yvonne's feelings on the matter, Elsa's safety came first. As you can imagine, this line of thinking REALLY messed Yvonne up.
This downward spiral basically continued until the Villainous System itself changed. Suddenly, Yvonne's entire mentality breaks down, as her being close to Elsa no longer seems to be an inherent danger to her. After that, she's mostly coasting by on inertia, no longer concerned that her presence is an immediate danger to Elsa, yet still paranoid and overprotective of her. She's basically denying her feelings out of habit, rather than from her mental coping, thus leading to some of the rather comedic moments we've had as of late.
Only for us to get to the Love Potion, and thus here we are.