I will say up front that I am on the fence. But IF this was a suicide, I can imagine it coming about like this:
Hypothetically, let's say CD was battling some kind of depression. She sounds like a very strong, capable, and kind woman - the type of person who might feel she should be able to cope on her own, and also not trouble her loved ones. Speaking from some experience with depression, I know it can occur even when life seems perfectly rosy. I am happily married with a loving family. But I have had times where I just wanted to crawl in a hole and be left alone for about a week. And it makes you feel guilty and confused because nothing is really wrong. It is also exhausting. So I know this is possible. And in such a state, the least little thing going wrong can just be way too much. Example: the cat pees on the carpet. You clean it up, right? But if you are depressed, it can literally seem like too much to handle, and you fall apart. I am not exaggerating at all. I have been there.
Continuing my speculation: Imagine that CD was having bouts of similar despair, and worse. She may have been muscling through her days through sheer will power and perhaps sometimes feeling like she couldn't go on. Another Monday rolls around. She heads for work with the familiar sense of dread, and life being a burden, that depression can cause. She realizes she forgot her badge and it's like the last straw at that moment.
Maybe she genuinely thought about going back to get it (hence the text exchange) but then it all just seemed too much. So she powered down the phone to think, and be left alone to her thoughts. She struggled mentally for who knows how long, and then somehow decided today was the day. Maybe she had been carrying the blade around "just in case" - to tell herself she had a way out if she needed it.
Maybe then she tried test cuts and decided that wasn't what she wanted to do. Then she cast about for options and made do with what she happened to have available. She may have walked and thought some more, at first heading toward her home (based on the most likely route to the culvert), but then ultimately choosing another road.
I admit I still find the culvert a sad and puzzling choice, if it was a choice. But I can clearly imagine the state of mind and the accompanying mental struggles that could have led to someone taking their life with so many contradictions along the way.