PK - I have found it helpful to send a text/e-mail like this:
I've been stewing over a subject and am too nervous to bring it up face to face. Could you do some re-search on subject A?
This has been my thoughts on the subject: (list them out).
Let's schedule a time in a day or two (or whatever) to discuss this further.
It has helped to keep from blindsiding my husband. I too find it extremely difficult to just bring up a subject out of the blue face to face. Things tend to get all twisted around, especially when I feel like I'm being attacked before getting my entire point across (and I tend to ramble when I'm nervous). I will write everything down, in detail mostly for my own benefit. I might give him a "cliff notes" version to ruminate over ahead of time.