I'm sorry to hear that you are having trouble with your girlfriend. That must be really upsetting.
So, from what I understand, your girlfriend has been piddling around with the idea of dating a man for a year now, but now she's actually dating him. She told you that she wouldn't date him, but then changed her mind? She knows that you are uncomfortable with him, but still dates him AND still expects you to be fine about it? Well, that doesn't really make sense, now, does it? She should either not date him in the first place, if she can't deal with the fact that you're going to be upset, or she should take the consequences and be willing to help you out, because you've helped her out by not slapping the veto rule on this guy, because you don't believe that's right.
I think that some people are just harder for us to deal with. It doesn't matter if they're nice to you or not. I've been really upset over some people, for the longest time, then years later, I look back and wonder what I was so worried about. Then, with other people, I feel calm and relaxed and something bites me in the ass.
How can you give a time frame for how you will feel? Especially when she couldn't even stick to a tiny two week time frame of not dating? Well, I guess she technically didn't go on a date (from what you've said) - but it makes me wonder how hard it is not to look on dating sites for two weeks when you have the rest of your life to look for that kind of thing. If it's taken a year for her to figure him out, why would it only take you a few weeks to figure him out? It doesn't matter how long ago she met him. She could have met him 15 years ago and started dating him yesterday. Then, your processing would have begun yesterday.
It's good that she tried to help you over the summer. But what? She thinks that some talking is going to make everything go away? Long distance relationships are hard. Especially when you are the one being in one country and then another country. Does she ever come to see you, or are you the only one with your life in upheaval?
I do understand that she can't keep reassuring you forever. But I do agree that we act different with each person and with each experience. Just because it took you a year to get used to her other boyfriend, doesn't mean it will take a year for you to get used to this one - because you are further along in your relationship now and it's a different person. It's like when married couples still keep bringing up arguments from ten years ago. You learnt from your mistakes, so move on. Instead of doubting you, she should trust you to work it out, because you're still with her and you never vetoed him.
It seems to me that she's saying she wants to help you but really she wants to help herself. It's boring and stressful for her to have to keep reassuring you, so she doesn't want to have to do it any more. To me, that's not really thinking about you. If she was thinking about you, she'd bite her tongue and say take all the time you need. If she said that, you'd feel her love and you'd probably be ok a lot faster. By making you feel like a burden, it's only going to make you feel worse. I think if she's reached the end of her limit or she's getting close to it, then it's fine to tell you about it. But what worries me is that she's reached her limit so fast. She's only had two dates with him. It doesn't matter what went on last year or this year. She's only had two dates with him. Maybe she needs to work on her patience, instead of you working on your insecurities.