I agree they are both fine. A more informal option would be to say "It isn't in your nature to be that way." You could also change that part of the second one.

I don't think there is a problem with changing them, but you will rarely ever come across this phrase in speaking or writing. Usually we say something like: "It's not in my nature to do so" or "It's not in my nature". I have never heard this before: "You don't have it in your nature to be so." But I don't think it's incorrect to say but extremely rare to come across. I usually come across: "You don't have what it takes to be so" and "You don't have it in you to be so".

They're both grammatically correct, but the first sounds more natural.