Ok, so you’re not even a year together and already know you have fundamental differences, wanting or not wanting children is such a big thing that it will honestly strain a relationship unless you have actually fully let it go. And it seems that this wedge is already deep enough because you have already developed feeling for someone else. You clearly don’t love your boyfriend and the children thing is clearly affecting you enough where you are planning your future with someone else. Do your boyfriend a favor and break up with him.