Girls -am I an Idiot? Did I Blow it?

I would find out why your ex cheated on you, and you may find it is the same reason your new gf dropped you. Cheating is never completely one sided, and if you are cheated on you need to understand that somewhere, somehow, you failed to satisfy the other persons needs in some way. If you can be honest about that, you will improve your chances in the future. Telling your new girlfriend probably had little to do with her leaving, but it may have reinforced some doubt she already had about you as a longer term prospect.