I think you have to find a way of assuring her that your freaking out was a completely normal reaction.
You have to get through to her that it wasn't your child you were rejecting but the IDEA of a child, an abstract one. You have never had a baby before (dunno what your experience of babies is) and therefore you were really worried and couldn't see it working and thought that maybe abortion was the best thing in the circumstances. You weren't actually thinking about what abortion means for you, her and the baby. It wasn't what you had planned so you were looking for what seemed like the best and easiest solution. It doesn't make you some kind of unfeeling monster.
Your initial reaction of happiness says lots. It was instinctive, but naturally when you thought through all the implications you began to panic. That is so completely normal. I think because you didn't initially freak, she completely misread the situation, had you instantaneously gone "OMG, what are we going to do, Nightmare on Elm Street" then it would have been different. As things began to sink in and the implications of what a child means, you began to worry.
Usually, when a woman presents with a positive pregnancy test at a Family Planning Centre, if she is adamant that she wants an abortion, best clinical practice is to tell her to go away and think about it for a week. You haven't had long enough to let this sink in. A week is a pretty short space of time and you are not being unstable. It shows that you trust her enough to share your deepest feelings, worries and anxieties about it with her. This is what happens in relationships. You make yourself vulnerable to another.
Being a human means being subject to human emotions, you can't turn them on and off like a tap and neither can you just respond the perfect way to everything. I tell you I was treated a lot worse than your GF when it happened to me and the only way I managed to forgive was to understand that his reaction was not voluntary - he was genuinely ****ing his pants at the prospect of having a baby. And it wasn't his daughter he was rejecting, merely the idea of a demanding baby.
Wish I could talk to her and help her see that your reaction is mild by comparison. She wouldn't always be waiting for you to change your mind because by the time she gets to 20 weeks it's a done deal, the NHS won't touch you with a bargepole re abortion unless God forbid, there's something terribly wrong. She has to put your former reaction to one side and focus on what you and she want now.
Also you say she's 2-3 weeks pregnant, that translates into 5-6 weeks in pregnancy terms. The pregnancy is always dated for clinical reasons from the first day of your last period. (Yes it's odd, it means that most women are always technically 2 weeks pregnant when they ovulate, but that's how they do it). If you look up the development of your baby, you'll see the heart starts beating from this point. Until the placenta kicks in at about 12 weeks, she's likely to be exhausted, feeling pretty sick and generally just bleurgh. The reaction she's having isn't rational. She can't hold some understandable initial panic against you, this is a massive thing that is going to change your life forever. For the better though trust me.
