Cheating is the worst possible thing anyone can do to someone, it distroys people. I am so sorry you are feeling like this, you will be angry for a long time but believe me it gets better. My partner of six years cheated on me too after a while she begged me to get back with her and I stupidly I believed everything she told me...I wanted to believe she was sorry and had changed but she didn't, she did the very same thing again to me. It broke my heart but it does get better, and BPD has some very good qeustions you need to ask yourself, especially are you staying because you don't want to be lonely, this is torture for you. I wish you all the best and remember to take care of yourself x