Probably one of the most useful apps I can recommend to people with bpd. I know you guys have probably heard this a lot, but it makes the WORLD of difference lashing out into my notes instead of my partner or friends when small insignificant things trigger my bpd. 
Writing out everything I want to say to him when I'm mad and hurt, then doing something else (watch a show, read a book, listen to music). I feel completely fine after about 10 minutes. I still might be annoyed but I don't feel the need to react irrationally (yelling, crying, trying to cut them out of my life). 

Please try and do this next time you feel upset or mad. I hate seeing posts on here about couples and friendships ending because the non-bpd person feels overwhelmed. My last relationship ended because of my inability to control my emotions and the fact that I would lash out. It's honestly a horrible thing. 