I am by no means a therapist or psychiatrist, so take what I have to say with a grain of salt. What I have to say comes from my own experience in treatment and from being around hundreds of people with various mental health issues.

It sounds like you may, in fact, have depressive issues. Sadness is a symptom of depression, but people don't always feel it when they are depressed. Depression presents itself in a variety of different forms. Anxiety is a symptom of depression. And by the same token, depression is a symptom of anxiety. So it seems as if you are experiencing more than just "mild anxiety" It could be more severe than that. Especially since Valium is quite a hefty medication to be taking for mild anxiety.

It's actually fairly common for people to have odd thoughts like you have just described. People often express fleeting thoughts of crashing their cars, jumping in to traffic, taking pills, jumping off bridges, etc. Generally, people fleetingly think it and then push it from their minds. For you, however, I think it became coupled with your anxiety and the anxiety took over and began to obsess on the thoughts and made it spiral out of control until your valium kicked in and knocked you out.

That is my theory and how I am perceiving it, of course. Your best bet would be to talk to a therapist and work out what's going on.