If she takes treatment when she has a flareup and you avoid physical contact when she does, that should minimize your chances of getting it.
However, if you're paranoid about it, you should know that it will never be a 0% change of transmission (even with condoms), given possible asymptomatic viral shedding too.
But herpes is what it is. For some people, it's not a big deal, for others it is. Yeah, it's a STI, but a large percentage of the population has it, even kids (whoops, kid kissed someone, now s/he has it on the lips). I mean, you get a few crusty vesicles for a few days/week, and it'll be uncomfortable or might sting a bit, but they will go away (and treatment can help make it less severe and last shorter). As for frequency, it varies a long. Some people never get a flareup, some every few years, some every month.
There's technically a difference btw oral herpes (hsv-1) and genital (hsv-2) in terms of severity (2 is worse), but they really are interchangeable. Someone with cold sores could give the milder hsv-1 genital herpes to someone else via oral sex.
Herpes is not a death sentence, and you might even have already been exposed to it if you've ever kissed anyone (or even a family member as a kid). If you stay with someone who has herpes, there's a chance you get it. Maybe, maybe not. But you run that risk if youbget sexually involved with anyone, it'll never be 0%.