Dreams are expressions of your feelings about your waking life at the time of the dream. Also, feelings in dreams are never disguised, so how you feel IN the dream is how you really feel about its subject in your waking life. In the dream, you feel "comfortable" about this relationship, but then he turns mad and you find it very scary.
It would seem from what you revealed about your waking life that you are scared about what you feel would result in a violent reaction. Dreams are often symbolic, so it's likely that in your mind you switched up these two men, so that in the latter part of your dream, the new romantic interest represents your current romantic interest... who you feel would be angry/violent about your straying from him.
The key symbol in your dream is cheating. Here's a link to its definition that explains it all:
dreamdoctor.com/dream-dictionary/definitions/cheating.html
Your dream reflects your feelings about your current relationship. It is not about the other guy, it's about what you feel is lacking currently and what you feel would result if you went looking elsewhere.
Be sure to let your boyfriend know what you need (guys aren't mind-readers.) If you can't get what you want (your dreams will confirm it), you owe it to yourself to find it. It's out there.
Good luck!