Does depression cause a complete inability to focus?

I've struggled with my inability to concentrate for many years, but lately it feels like it's getting to the point where it's both ridiculous and sometimes dangerous. I can't concentrate even when I'm very interested in the thing I'm trying to concentrate on. A few days ago, I went to Universal Studios, and I kept finding my mind wandering IN THE MIDDLE OF THE RIDES/SHOWS. One moment, I would be trying to take in the scenery on the ride to understand the plot, since most rides at Universal are centered on a plot, and the next moment, I would catch myself off in a daze thinking about how good chicken tenders sounded or something I wanted to text to a friend when I got off the ride. Then I would get frustrated with myself because I paid money to be there, and I didn't even really experience the attractions because I was too busy in dream land thinking about food or whatever.

I'm also finding it nearly impossible to read books these days. I used to be an avid reader. Now it takes me 6 hours to read 30 pages. I have to re-read sentences over and over because my mind wanders and I never really take in what I just read.

I've been tested for ADD/ADHD and have also been on medication for ADHD despite passing the test. The psychiatrist said I passed the test, but he would prescribe me the medication anyway to see if I found it helpful. When Vyvanse didn't work, I was put on Straterra, which also didn't help. In fact, both medications made my problems worse, the Vyvanse because it made me very jittery and the Straterra because it caused me to fall asleep off and on all day long and have such vivid dreams I couldn't discern them from reality. So I don't think it's a pure attention deficit thing. Could it be a depression thing? I'm so frustrated and feel like I'm completely losing my mind half the time. :/

I have found that about reading @wanttolivebetter. I will sit with a book on a single page and keep re-reading the same paragraph many times over and over again, but my mind wandering and not there. I would stay adamant and have managed to finish books, but it would take me a month or 2 months to finish one and I don't quite have the same reading experience, because of the breaks I took in reading it.

And like you say, I have had my mind wander in between listening to someone or a team-member describing something. And then I would have to apologize and say please can you repeat that last bit? or I would say something to make it sound less nasty than I wasn't listening at all.

I think it is a common thing.

In the past, when this was affecting me in a really bad way, I signed up for a meditation course and also practiced it for a good few months and found it helped me a great deal.

I am now struggling to get back to meditation.

Mindfulness or meditation, its a way to slow down racing thoughts or observe thoughts. Would totally recommend trying it.

It does for me - had a terrible week, trying to get some work done and taking forever just to mark a single paper. Feeling maybe I'm not well enough to stay in work but then I know if I allow myself to take time off I will go downhill very fast - at least work gives me an external focus even if my ability to sustain it is weak at the moment.

I can so much relate to this. This is the primary reason, why I get up and drag myself into work, every single day.

In addition to, having bills and loans to pay of course

When I read, I either read short articles, or I read a piece or book in short sessions over the course of weeks.