I'm working on a war novel (sci-fi). My initial plan was for my MC to start out with a very clean language, almost comically clean, and as the plot progresses and the situation gets more FUBAR, his language would get more colourful. Other characters were to use different levels of profanity, based on their background and time on the front. For the MC, the change in the language he uses was supposed to reflect the effect of the distance from "normal" society and the protracted warfare on his psyche.
Trouble is, I do not use dirty language in my day-to-day. At all. To the point that I'm known for it among my friends. So when I come to writing profanity, it feels awkward. The expletives don't feel as natural as they should. They stand out and feel deliberately inserted rather than being part of the speech. And at the same time, they feel toned down, like the character should have used something stronger, if they were to use a strong word there.
How can I write this kind of language, that I do not talk, not even as part of my internal monologue?