A chemical reaction

In a relationship, you have elements called trust and honesty. To me, those two elements have become that hardest to keep ahold of. Maybe because they've finally turned into gas, instead something solid and easy to hold onto.
Halloween keeps replaying in my head. My dreams include those moments, but overexaggarated and with unicorns and fairy floss, because, as I said, its a dream. But that moment, in my head over and over again, allowed it to boil. Now it's steam, and hard to grasp onto.
Of course, this chemisty metaphor may be all wrong, I haven't done science since grade nine.
But, back to honesty.
Halloween wasn't dishonest. But I can't trust him to be honest anymore. He doesn't tell me how he feels as often, because he doesn't want my mood to go from positive to negative. I don't understand him, so this may be all wrong.
But it's annoying. And it hurts.

So my question remains;
How can I be in a relationship if I can't trust the person?