There seem to be several causes of loss of focus in depression - I have identified that all of them impact on my functioning at the moment

1) Distracting thoughts (including past memories, current worries, suicidal ideation) - this affects me a lot - my thoughts seem either to spiral in many directions till I wear myself out when I do have energy, and when I don't it's just fogginess

2) Lack of energy due to loss of sleep and extreme effort keeping going (distracting thoughts also take energy)

3) Apathy and loss of motivation/volition

4) Cognitive effects - particularly on working memory which makes it hard to keep the thread, contention scheduling (ie filtering out possible options to maintain focus) which in turn seems to be affected by problems with executive function (overriding impulses), and problem solving affected by mental fog and lack of clarity

5) What is sometimes called 'psychomotor retardation' is also an issue - physically I am also slower and very clumsy, everything takes more effort, I find it's very easy to knock things over or drop things