Sorry to hear all that.
I'm afraid the hurting isn't going to stop any time soon and you need that to allow you to grieve and lick your wounds a little. Is there any chance you can get away for a bit and have a little time out on your own? It's always very pressured when something like this happens and it adds to the sense of distress and loss.
With regards to whether you can believe her, I'm afraid that's up to you. Your trust has been severely damaged and without that it's rather difficult to carry on a relationship. I think it's a bit too early in the game to assess the amount of damage that has been caused and what the answer will be in the long-term.
I don't think there's any need to feel a fool, I don't think you need to be that hard on yourself, you've done nothing wrong, you're just done what we all do when we love and trust someone which is bumble along unsuspectingly, it's rather a dent to the pride when you discover otherwise but it doesn't make you foolish.
Just look after yourself in the immediate future, ensure you're eating and drinking and if possible, take some time away from the actual situation to compose your thoughts and yourself.