Based on life experience, I think the only sexual history that should be shared is a history of veneral diseases, marriage history, and any exes who are still around or who show up. A history of cheating is also valuable to know, because that not only tells me that it can happen, but, also based on experience, likely will again.
My dude, you cannot unknow what you now know. I respect you reaching out for advice about how to deal.
Inadequacy is a common male response to such admissions when they have less experience than the female. I don't know why that is. Maybe it's a biologically-generated response to your sperm having to compete with other sperm for a limited number of eggs, and if your sperm hasn't had contact with the same number of egg producers if not more, then your genes, not you, are threatened by the competition. And males often freak over female bi-sexuality -- it is both turn-on and threatening competition.
My advice is to take some time to evaluate her overall character based on her consistent behaviors over the past five months, let that simmer for a bit, and then factor in this admission to see if it has any relevance with regard to who she has proven herself to be. Think about the context of her telling you -- was it because you've reached a new level of emotional intimacy and she was being trusting and vulnerable? Did it feel inappropriate or weird timing-wise, sending up red flags? If a male friend admitted a similar history, would you admire him, judge him, or not give af at all?