Carol, I don't know what kind of phone you have, but my Samsung Galaxy has an icon in the bar right above the keypad. It's a little arrow pointing to the right. When I type a name or word and then press it, it allows me to enter that word in my predictive text. I discovered that a couple of years ago when I was trying to abbreviate my grandson Tyler's name to Ty, and it kept trying to change it to "To".
I'm not real smart phone savvy, but I text a lot.
Maybe your phone has something similar to instruct it to change Dean back to Sean.