I think that a female partner sometimes hits too hard because:
A) They think they don't have to hold back because you are both female or
B) They are scared shitless because they assume YOU won't hold back because you are both female.
If she is gassing out, she is probably anxious and tight. That sounds to me like she isn't just out of shape, but afraid. People who are afraid tend to hit too hard in a misguided effort at defense...(in the long run it just makes it more dangerous for them since we, as the offended partner, will likely just hit back harder in frustration as you did!)
Perhaps as she gains some confidence, she will mellow out.
I think there was already great advise dispensed for asking her to try hitting a few times to calibrate the power of her striking, asking the coach to intervene or set you up with a different partner...but I think you could also just stand next to someone else and grab a different partner for yourself as soon as the pairing up starts. It's no fun to get stuck with the same partner for every class...Even when it's a great partner.