Someone once asked...
Someone answered with kindness...
Your relationship with your boyfriend does not have anything to do with your new fancy. If you dont want to be with your boyfriend, break things off with him cleanly before you start something with someone else. If you already started something with this new person. Tell them to wait for you while you break it off. That will give you some space to think though what you say. I suggest not making up a lot of reasons for why your relationship isn't working out. Just explain that you want to move on.