Move on and preserve some self-respect for yourselves. It is difficult at first, but it's worth to overcome plus it's a good motivation for self-improvement. Despite your strong feelings towards him/her and his/her current relationship, if you pursue it seriously, can you truly say to yourself without any sh3er of doubt that what you are trying to do is right?
Life advice: Don't expect things to go better when you know your action is wrong in the first place.