i havent even came close to reading all the responses to this, and you appear to have reached a personal resolution
but i think there is a good possiblitilty she is already cheating on you, thinking about it, or just wants to end the relationship but for some reason wants it to be YOU to break it off and not her
you say you have complete trust in her, and that may be so, but is this your first relationship or your first "serious" relationship? it's very easy to put complete trust into someone, and you don't understand what that can mean if they actually DO break that trust - total devastation.
if she cannot and will not come to any compromise with you after sitting down and having a very reasonable question/answer period about whats going on, what bothers her, and what you think about it, you both need to decide if you are actually doing anything wrong or if s he is simply being insecure. You need to reassure her of your stance on the relationship i.e. you seem to feel that you would do anything to make her happy etc
not sure if all that makes sense, but there is alot of practical advice in that (in my opinoin). a woman should never have the absolute say in the relationship (neither hsould the man). you should both feel completely sure and confident in having a 50/50 relationship or you are going to run into some major problems.
the simple fact alone that she not only gave you an ultimatum (she would leave) but a very SOON ultimatum blows my mind. she may be a great girl, but for her to do that means there is some serious negative shit floating aruond in her head. you either need to straighten her out, or if she doesnt compromise or realize that SHE may be creating problems as well, the relationship will end. maybe not today, or two years, but you are going to be the one fucked over.