As someone with ADHD, focusing can be really hard, especially if it is something I've been working on for some time. I've yet to find the perfect app/answer for me. I've found Monday.com to be quite good. While it's not an app specifically to help people focus, the freedom to organise your work in a way that works for you has really helped for the times I do start to lose focus. I know I work differently to many people but Monday has helped.
I used a completely personalised and tailored Monday board to organise our Product Hunt launch and I think it's really helped me plan today's launch!
Greenspark