It happens to me as well a lot. Usually with chronic technology use, especially being inside a lot as well can lead to this. Also, working hard means different things for different people, so it might be that you working a lot harder I would recommend maybe getting some fresh air and doing something small, maybe 15 mins of work as a start, then take a break and do a bit more. You could set a pomodoro timer maybe? Don't worry too much about maxxing out productivity for now, getting a little bit done is still amazing.
A lot of times people get distracted because they're sad, and its easier to escape by doing something else. Don't be too hard on yourself.