Hold up! You are emotional right now. Likely so is she. I've known LBSs that found their WAS in bed with another person, and they ended up reconciling and having a very long happy marriage afterward. So just slow down, calm down, breathe, and try to think rationally about all of this.
You seeing her with OM, while upsetting, didn't change anything. In other words, if you had never been by them and saw them it would still be going on, but you wouldn't have known!
So take time to process. There is no need to rush out and file for D right now. We have a saying around here: never make decisions about your sitch while you are emotional.
Also DJH, you are going to be fine no matter what. Try to remain calm.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018
