Hi boatyard I don't have any experience with the app but it sounds interesting.
Mindfulness seems to be very much "in vogue" at the moment and I'm certain it must help a lot with anxiety and depression, several Gransnetters have posted about using it in past threads, so perhaps do a search.
I don't suffer from anxiety or depression but I feel very sorry for people who do, it must be awful if that is your constant companion.
Back to mindfulness - and I'm not knocking it - as I said I am certain that it works, I sort of think that I do mindfulness without realising it because I can "lose" myself in baking a cake, or sewing something, or working out a complicated knitting stitch (well complicated for me, because I'm not very good). Or going for a slow walk and letting my natural curiosity have me taking everything in, a bit like a dog sniffing its way along a path. So I always think of that sort of thing as my own version of mindfulness.
I think it was the author Marian Keyes who wrote a book about baking her way out of a depression - you can't help but being mindful when you have to pay attention e.g. to a recipe.
Hugs and good luck.