In most of our writing assignments, we have a backspace button. We wouldn't draft as many work emails without the ability to edit quickly. We wouldn't give a toast without running it over in our heads or with some index cards to prop us up.
It's no different with a meaningful note.
Grab a scratch sheet of paper and jot down some bullet points and a rough draft. It may sound like extra work, but I find it actually helps things flow a lot better. A dry run gives us permission to get the generic sentiments or choppy thoughts out of our system.
It's easy to write "thanks for everything" and then realize that's not what I actually want to say. But better to cross out a line on the back of a receipt than on the card I just bought.
This isn't necessary every time we write a card, for sure. But next time you're struggling to get the words out, give yourself a low-pressure take before writing the real thing.