UNFAITHFUL

Someone out there tell me how can the one you LOVE so much be so unfaithful? we have children together and that was not enough to keep their LOVE true and Faithful to only me! At the moment just feeling sorry for myself I guess, but I'm the type of person who believe's that I truely did'nt deserve this, nor our children. How do I get myself out of this shet!? My partner keeps telling me it was a mistake!? But was it? someone out there HELP ME!