What are other sayings or expressions for "I can't wrap my head around"?

Solution 1:

You can get or wrap your head, mind, or arms around problems (often unsuccessfully):

• The oldest of these expressions appears to be to “get one’s head around” something, a usage that the Oxford English Dictionary has been tracking since the 1920s.

• The OED defines the expression and its variants as “to master or fully comprehend (a subject or fact), esp. despite initial difficulty or reluctance” or “to come to terms with (a situation).”

(grammophobia.com)

Get your head around something :

• (informal) to be able to understand something (usually negative):

  • He's tried to explain the rules of the game dozens of times but I just can't get my head around them.

(Cambridge Idioms Dictionary)

Solution 2:

One of my favorites is "bake my noodle."

In your example:

"But what still bakes my noodle is why she behaved like that."