I can't decide if I want to drop her a line.

Yes, it does sound a bit awkward...🙂 What about: "I can't decide whether to write to her."

The expression "in terms of" is common in mathematics for statements like "solve for x in terms of y and z". Besides such scientific uses, I can't imagine any situation where it is needed. In your sentence, it is bloated verbiage that can simply be struck out.

I’m struggling with whether to call her.

I would write something like, 'I'm in two minds whether to drop her a line or not.' In two minds is an idiom that means I can't decide or I'm not sure.