Welcome to the STD forum. I'll try to help.
You describe a very common situation in genital herpes -- indeed your story is classical, exactly the same sort of case that STD experts use to teach medical students about the disease. The main thing is that her newly diagnosed herpes doesn't necessarily mean that either of you has had other sex partners since you have been together. If you are otherwise confident your partner hasn't had sex with other people, you should trust her about it. And she should trust you on the same score.
Your partner may have been infected with HSV-2 for a long time. Even if her recent genital sore was the first time she noticed anything wrong, it could be a recurrent outbreak not her first infection. If she had multiple sores, rather than a single one, or if she has had other symptoms, like inflamed lymph nodes in the groin and sometimes fever and headache, the probably it's her first infection. If it is her first infection (and if there have been no other partners), then you can expect your blood test to be positive.
There is roughly a 50% chance your blood test will be positive for HSV-2. If so, you might have either caught it from your partner, or perhaps you were the source of her infection. It your test is negative, it will show you were not the source of her herpes, and that so far you have been lucky in not catching it from her. This is a common situation. Among heterosexual couples in which one person has HSV-2, the rate of transmission to the other partner is around 5% per year. In other words, in such couples, only 1 in 20 will experience transmission to the uninfected partner (these data are with an average of unprotected vaginal sex 2-3 times per week). You could easily have gone uninfected even after 7 years of regular sex.
Time between outbreaks? In terms of symptomatic outbreaks, your wife might not have another for several years. But if she has very mild ones and keeps on the lookout for mild symptoms, she might have them as often as every 1-2 months. Regardless of how frequent she has symptoms, she also is having asymptomatic viral shedding; the virus is present (without an obvious outbreak) up to 10-20% of the time. Her doctor didn't confuse your wife's herpes with anything else; as the doc said, her lab test shows "she has it".
I think those comments answer all the questions you asked. Please return with a follow-up comment to tell me your HSV blood test result and we can discuss it further. But don't be too upset about all this. As I said, this is a very common situation among couples with herpes and there isn't necessarily any implication about sexual fidelity.
In the meantime here are some online sources with accurate information about genital herpes: the Centers for Disease Control and Prevention (www.cdc.gov/std); the American Social Health Association (www.ashastd.org); and the Westover Heights Clinic of Portland, OR (www.westoverheights.com). Full disclosure: Dr. Hook and I serve on ASHA's board of directors, and Terri Warren, moderator of MedHelp's herpes forum, is owner/director of WHC.
You'll undoubtedly have more questions, but before asking them here, please read all this information carefully and also wait until your own blood test result is available.
Good luck-- HHH, MD
In addition, take a look at the MedHelp herpes community forum, where the moderators are highly qualified herpes counselors. However, I suggest wait until your own test result is available. Until then, they also won't be able to answer some of your key questions.