Boyfriend has a crush

Last night my boyfriend went to lunch with a girl he met last week. I was already uneasy when I found this out (he's cheated once before but I took him back because he was so remorseful). He informed me that she knew about me and that he told her first thing that he didn't want to cross any boundaries. He told me not to worry and that they were just friends but later that night I couldn't shake the feeling of the conversation being unfinished. I asked a few more things and he eventually said that he liked talking to her and that he might have a crush on her and, worst of all, he almost kissed her. It doesn't hurt so much that he almost kissed her..it hurts more that I think he likes talking to her more than he likes talking to me.. He said we were done that night but I'm in the middle of exam week and I can't handle anymore stress so I literally said "no. not today." The atmosphere has been really weird around us today and I don't really know what to do. He's been telling me that we're going to get married and have kids and even got me a promise ring and gave it to me on Sunday. He can be impulsive with his words at times and I don't want to lose him if this is one of those times..What should I do?? (please don't say leave him because I got enough of that from my really unforgiving roommate).