Having kissed him, did they go on to have sex?
If not why not?
If it's really true that they kissed and went no further then I'd see that as a massive positive.
If they went on to have sex, who would she rather be with now?
If it's him, send her off with your blessing.
If it's you, why does she want to be with you now and not him? The answer to that may be positive enough that it's worth staying with her. Although you should only proceed with marriage if you're as sure as you can be that her straying days are over.
And how do you feel now? If you put aside this "kissing" and keeping in contact episode (tricky to do because it's such an emotional topic, but try your best to think about this logically) are you confident that you'd be able to find someone "better" - someone that you'd be happier to share your life with - than her without too much difficulty? If the answer to that is yes or probably or maybe you should dump her and get another girlfriend. In fact you should have dumped her before the "kissing" episode.
There are different spins you can put on this kissing and staying in contact episode. Probably the most positive spin is that you believe in free market economics. You believe in people having a choice. This kissing and keeping in contact thing was information for her. Information is a good thing. You want her to be with you because she wants to be with you. And not because you've got her trapped in some way. And vice versa for you.
It's like if you're a steak-house and your biggest customer goes off to McDonalds, would you be worried that you've lost that customer forever? Or would you be pleased that they had the chance to compare you against a lower quality product?
And is she the equivalent of McDonalds or a steak house or a Michelin 3 rosette restaurant?
If you're working 70 to 84 hours at something that fulfills you, that's great. However in future aim to fit in some high quality time with you girlfriend each week. Even it's only a total of 3 hours. Make it a fantastic 3 hours for her. Which doesn't necessarily mean having to spend a lot of money on her because there are plenty of fun, interesting, pleasureable things you can do for free or nearly free.