Sorry you're struggling with this. I'd basically tell him what you wrote here--could print it out and hand it to him if it's too hard to say. Or say something like, "It feels like you've jumped to conclusions about me when I've only given you information about one area of my life. And now I'm reluctant to open up and feeling guarded." If you use "I feel" statements, then it's less accusatory. Can I ask how long you've been seeing him?