Seeing things one at a time the way WK presents them is really unfriendly and difficult to track given how full of exceptions Japanese counters are. I found it helpful to look at things in a chart so that you can visualize the patterns and see how some counters relate to one another. Here’s an old one I drew up back when I was having the same problem in the early levels.
You can see that month/people and days/things largely follow the same prefix patterns with a few exceptions. Yellow indicates that an alternative reading for that number gets used (e.g. し instead of よん), green indicates where there’s a variation in a pattern (e.g., むいか doesn’t map perfectly on to むっつ the way みっか maps onto みっつ), and red highlights weird, exceptional readings that come from total left field that you’ll just have to memorize. Oh, the blue is just there as a visual reminder that people/things have the same prefix pattern for the 1 and 2 counters (ひと and ふた).