You're on acyclovir for chemo? I'm assuming that you are taking it to help prevent mouth sores associated with the chemo?
This is a tricky question for you. Ordinarily, there is about a 2-3% chance of a male getting hsv2 from a female over the course of a year when the female is using daily suppression, like Valtrex (this assumes sex 2-3x a week).
However, you are on chemo, and your immune system may be compromised because of it. I can't say for certain because I don't know what kind of chemo you're on, what kind of cancer you're treating, etc. You should talk to your doctor about that.
There is some thought that acyclovir or Valtrex may help prevent herpes infections when taking it prophylactically, but the science is still out on that.
I have to say that I'm concerned about your girlfriend who didn't tell you that she has hsv2 knowing you were undergoing chemo. That's a pretty big health risk she took with you. Your chances are probably low, but she should have told you.
Talk to your doctor about your immune system status, and make a decision from there. You may want to use condoms along with the valtrex while you're undergoing chemo.
Best of luck to you.