All of Text changes

[Mgmator]

Sometimes when I use the program, I can edit just single letter/word within a text box, other times the entire text box will change. This means if I want some words/letters one font and others not, I have to make space for that word in one text box and then create a separate text box for the font that's different -- making something that should take a few seconds take 10, 20, 30 minutes or more depending.

Is there some sort of setting I can change so that the entire text doesn't change?