I am really afraid to date again, because I don't want to trust someone and get my heart broken. How do I keep it casual, and when is it okay to let my guard down?
I honestly think your already doing the right things for yourself, and advise you don't change a thing.
That your able to let them go, when the fun stops, is a plus on your part.
You just have to know how to cope with your feelings, but I think your judgment is dead on, as it is.
You have those feelings because your human, and you care for yourself. Your fear protects you, and you listen to yourself. That will take you far in avoiding games, and deceit, and BS.
Keep doing what your doing, and a real good guy will eventually appreciate the fabulous person you are.