@MaidOfTarth,
I do hope you realize it's the middle of the work day for a lot of people. More will respond once the day is ended.
Anyway - have you considered counseling? For both of you, if she'll go, or at least just for you. For I do not think it is enough for just her to get help.
Of course it bothers you. Of course you're concerned about trust. That's pretty dang normal. I would even venture to say that the August incident(s) might not even be too much of an issue, seeing as things were so new. Was there already a clear understanding of exclusivity in the relationship? Because if there wasn't, then it might not even be cheating. December, though, is a lot more problematic.
So - you need to learn to express your needs and expectations, clearly, assertively and kindly. This will be helpful not only in this relationship but in other aspects of your life. Furthermore, it will be of help if the relationship ends.
You could also use some counseling about your choices. Perhaps you are drawn to women who you feel you can rescue, or fix. Neither is a terribly good dynamic.
In any event, it is for this precise reason that therapists exist.