I was thinking the exact same thoughts as you 8 months ago. I was also in the same situation in terms of having sex & relationship length. I wondered a lot about what it would be like to be with someone else. I decided to end it but didn't like my choice a day latter. She made the decision final to end it for good. It is very hard to put 2+ years of history behind you & move on. I would say stay with her if you truly love her and you two get along well. If you were fighting a lot it might be a sign to move on. Who knows she might want a 3some some day.