I have someone who was killed a couple of years ago in a crime (a child), I know it is not the same but some feelings possibly are. The helplessness is one of the worst but so is the anger at the perpetrator and of course the sheer grief at the unnecessary loss of her life and future. We are still in the court process.
Not even knowing what happened has to be its own special kind of he77 and I hope you have people who help by listening at least. I can identify a little bit with the agony of not knowing what happened because the imagination can take over as can anxiety and more. While we have a perp in our case, he never came clean about the truth of what he did and I have to shut my thoughts off because I cannot handle them at times.
I am always here to listen if you ever want an ear. I do know what it is like to feel alone on an island. Most people have not went through such things, thank goodness, as I wouldn't wish it on anyone or on my worst enemy. A person I met online has actually helped me a lot, sharing it with a "stranger" one would not think would help but it is a difficult thing to discuss in our family as the pain is so raw it just brings up those feelings for each of us if we do. Another big help was a high school class mate who I was never really close friends with in high school. She reached out and I found her helpful and easy to talk to, she also had a job that enabled her to answer some questions for me. One never knows who will be that angel they need sometimes and I do my best to be that ear for others as well.
I don't know why but after it happened, I first got into politics (why???) and then into true crime. One would think that would be the last thing that would help but for some reason it does. I don't know if following a case through to justice is what it is or something like the fact that I feel I am helping (even though I am just sitting on a keyboard) and it helps me feel as though I am doing something when I can't do anything to change our case, bring her back or help to find someone. It is heartbreaking but it does absorb some of my attention by praying for and hoping to help others.
I hope you find your daughter and I hope I worded things right here. In no way do I mean I know what you are going through as I don't, I am simply reaching out. I may be able to identify with some of it although the situations are different, and I sure can listen.