<human>: Can you condense the text into a few key points?

Told my current girlfriend (official 2 weeks, but seeing each other for 4 months), about a girl I was seeing a bit when we first met back in October. We spoke about it about a week ago because I didn't want to keep it a secret anymore now that its official.

Met this girl a week before I met current girlfriend, and this girl moved away on exchange in December.

GF isn't too happy about it and is feeling pretty upset about this situation, but it happened before we were exclusive. She says that this has tainted her view of me, which really hurts. Ive changed, and know that I want her and Ive been reassuring her this, yet it is only temporary until she gets sad about it again.

I feel really shitty about it/not telling her earlier, but I cant change the past and want things to be great with my GF again.
<bot>: Current girlfriend is upset/sad about a past casual relationship that is affecting the relationship. How do I make her feel better/assure her that this is what I want? How do you get past something like this?