I'm only at 22 days and I have been using it for about 4 years - sorry! - I think it gets a bit broken with a monthly pay cycle, as it's designed for Americans who generally get paid once every 14 days.
Or we are just rubbish at this
maybe. What I do find really motivating is the net worth graph, and other graphs. If you've got a computer or tablet, look at it on the desktop site as that is really good and if you use Chrome, install the extension YNAB toolkit once you've got used to the basics. It gives you more reports and some other options.
I would urge you to assign that last £100. If you don't know what to put it into, put it straight into next month, or Christmas/holiday, or start an emergency/unexpected expenses fund and stick it in there. I tended to find that about £100 a month was the right amount to put in that. We used it most months. After about a year, I'd collated all of those expenses into their own categories - so we have stuff like car tax, which is yearly, bank account fees, DS1's school supplies etc etc, but also things like "phone replacement" "appliance upgrade" and I put a goal on them with a date and rough amount. But don't do that yet, a catch-all emergency fund is more useful as you'll be part way through the year for some of your yearly goals and that makes it harder to catch up.