This is something that's bitten me as I've gotten older. I used to be extremely accurate, but keep making this mistake... most often with space actually. So I end up writing 'th ebird' instead of 'the bird', for example. Also interested to hear OPs secret here.
Maybe not putting pressure on this muscle memory to perform accurately, so a lack of intentional practice. We're not really punished for typos with spellcheck and IDEs picking up a lot of the heavy lifting, so there isn't a lot of mental pressure to perform well here. Or it's simply aging related decline. It may even be a waste of time to worry much about it.
