“I’m too busy” usually translates to “it’s not on my list.” Be honest about the list.

Busy is usually a ranking rather than a capacity. Time gets found for whatever has been prioritized, and the language of busyness is a polite way of not saying that something came lower down, which spares both people an awkward moment. There is nothing wrong with the ranking itself. The dishonesty is in describing a choice as an impossibility.

Say the accurate version once this week, kindly, which is that you are not going to get to it. Use it on yourself too, about the things you keep claiming you have no time for. Then decide whether the list is the one you want.