My boyfriend has been lying to me! What do I do?

[Lilac]

So, I did something very bad the other day.

My boyfriend told me to go on to his Facebook to change some privacy settings because he wasn't able to get to a computer. As I was doing so, a chat popped up with one of his friends. His friend only said hi, but I could see the previous messages and I saw my name. Curiosity got the better of me and I read that particular message.

The general idea of the message was his friend inviting him to smoke/drink at his house. His friend said, "Other people will be there too. What if [my name] finds out?" My boyfriend replied, "Ohhhhh shit. Dammit you're right! What now?" His friend went on to say, "Don't worry, I'll tell everyone to keep quiet." My boyfriend thanked him.

Now, I knew he was planning to hang out with his friends and that plan never actually went through (something came up). Also, him doing drugs/drinking has been a big deal. He knows that I don't like it, but I have never told him to outright stop doing it. It's his life and I have no right to restrain him. Even then, he promised me with no prompting from my part that he would stop smoking/drinking. He said that he did not want to 'contaminate me when he kissed me'. I told him he didn't have to do that, but he insisted.

Even a few days earlier, his friends were smoking something and he wanted to try it. I didn't stop him, but he could tell that I was a bit disappointed and he apologized profusely and said "You know I stopped. I haven't done anything in months. I promise."

Now, seeing that message just made my heart sink. I have no problem with him doing whatever he wants. The fact that he lied to me hurts me so much. I tell him everything I do, even mistakes I made that he would not have found out about otherwise. I do not want to confront him, as I was not supposed to be reading his messages anyway and the 'hangout' did not occur anyway.

What do I do? I have never felt this guilty, but at the same time I have never felt so hurt