I agree with Claire, whose suggestions are perfect. I'll add a few more: I'm still sitting with it. I'm still making sense of it. I'm still unpacking it.

I'm still coming to terms with it. I'm still trying to understand it. I'm still trying to get my head round it. It still hasn't sunk in.

Here are a few alternative ways to say "I'm still processing it" depending on the tone or context: I'm still trying to wrap my head around it. I'm still coming to terms with it. I'm still figuring it out. It's taking me some time to absorb it. I'm still thinking it through. I'm still digesting it. I'm not sure how I feel about it yet. It's still sinking in.