"Lied about cheating" not to me but to his friends

My boyfriend and I have been together for nearly 6 years and have lived together for almost 3 years. Last weekend we went to a party at a friends house. He didn't tell me that he planned on doing cocaine there but he did. I don't really care that much except that it scares me and I hate it when he doesn't tell me about stuff like that. There was also a girl there who was working with him during a one month interview/internship (she was trying to impress him and his coworkers). All of his iMessage texts show up on the iPad that we share. The next night I opened the iPad and found that he texted an old friend bragging about how he cheated on me with a "smoking hot" girl. I confronted him and he said he was just trying to impress this guy.

You will think I'm crazy for this but I actually believe him. First of all, I was with him for the whole night. He said all that happened is that he signaled for her to pass a pipe but she came over to blow smoke in his mouth. He used to be a big partier but ever since he turned 30 he occasionally tries to prove that he is still capable of partying hard. This only happens once or twice a year and its normally just with drinking and staying up till 6 am so I don't care. Plus I'm 6.5 years younger than him and I still like to stay out late.

He truly feels awful about this. I made him text the friend he bragged to saying he was joking about it to try to sound hardcore. The other thing is that hooking up with girls is something that really impresses this friend so I think my boyfriend was trying to make him jealous of our life here. This friend got the job that my bf really wanted and my bf was so upset after that he was on the verge of needing psych intervention, which is extremely out of character. I think he really wanted to show the friend that he should be jealous of us for being in Miami and partying a lot.

I have pretty much already decided to forgive him but it has been really difficult for me because the text was so jarring to read. It was also so degrading to have something like that out there. And of course in the back of my mind I still wonder if he did actually cheat. The friend he told is part f a community that we are also a part of and people love crazy gossip so now I'm worried that everyone will know and think less of me.

I guess I don't really have any specific questions but does anyone have any general advice? Am I stupid for believing him? He keeps saying that it would be so different if anything did actually happen and how he just wishes he had never sent that text. He also seems like he is hurting almost as much as I am. He didn't at first until he realized that I didn't trust him anymore.