the epitomy of distrust?

What do you do when it's miraculously and steadily getting harder to trust your partner? Just some things that have happened over the last 24 hours were my girlfriend emailing her ex to see how he was doing, she asked me if I think she would ever cheat on me and then proceeded to compliment me on how good of a boyfriend I am, and then proclaimed she was going to change her password for her email account. I just checked; she did. She wouldn't have changed it if she didn't have anything to hide and no matter how much she tries to reassure me, I'm just going to be feeling this way until I find out the truth. You can count on it.