Mood swings that last for weeks or months sounds like bipolar, not BPD. I'm not trying to suggest you have that because I obviously don't know you. I just wanted to point that out.

My mood swings last anywhere from a few minutes to about two hours. It's a very quick emotional trip. I've been dealing with them by, well, being used to having rapid, extreme emotions and knowing that they last so short. You see, if I'm super depressed and wanting to SI, I tell myself to wait a few hours because I know the emotion will have changed by then. I also challenge emotions/thoughts all the time.