Everyone has been there, if they haven't yet they will be eventually. It sucks, but it is a part of life.
One important thing to remember is to never fully believe that you've found "the one", not just because it is a false concept and there is no such thing as "the one", but also because when you let yourself believe that, you start to slip on your game, which never really ends even if you've been married for 20 years. The good news is that there are literally thousands, maybe millions of women who will fit you as well or better than the last.
Now is the time to move on. Start by focusing on other things: hobbies, friends, family, exercising, meeting new women, etc. I'm sure you know the drill and have heard it all before but it doesn't help until you accept it and put it into action. It takes time to move on, so don't get mad at yourself when it doesn't happen overnight, or next week. One day you'll wake up and realize how much better off you are without this woman in your life, perhaps hard to imagine now but it will happen.
For now, forgive yourself for trusting her and forgive her for letting you down. Accept that she is gone and it is for the best. Accept that one day you will find a better woman than your ex and you'll look back and realize that the old had to end for the new, better to have a chance.