Basically my SO has a mother who is extremely controlling and constantly demands answer to every question with a 3 word paragraph on what he was doing. She is a human surveillance camera I’ve been bothered about this for almost the entire 2 years we’ve been together but I love them with every fiber of my being so I put up with it (she use to put this behavior on me and recently stopped after my SO did talk to her about it). Now the new issue is we were in the car talking about their friend’s mother and they referred her as “super controlling and too much” (for calling him over 4 times while we were at the gym) and I simply told them “do you not see the same behaviors your own mother does?” And they immediately said “no things are better now. She’s not like that anymore.” and I just snapped.. 

Mind you I am a quiet bpd so my rage NEVER is towards the humans I care for. But this time I couldn’t control it, “NOTHING HAS CHANGED YOU JUST TELL HER WHAT SHE WANTS TO HEAR BEFORE SHE ASKS. YOU TELL YOUR GRANDMA FOR FUCK SAKE SO SHE TELLS YOUR MOM TO PREVENT HER FROM CALLING YOU. YOU JUST FOUND A NEW WAY TO PLEASE HER NEEDS.” After that they went silence and simply said “yeah.. guess you’re right. Nothing did change.” And I feel like the worse person on the planet. I plan to apologize after venting now that my head is clear. I know it’s not my place but their mother constantly butts into our relationship and I’m sick of it. (To the point if they don’t respond to her. She will call me.)

I need advice please to prevent this from happening again and finding a healthy way of expressing this without it being towards them. they really care about their family despite how controlling and crazy they can get.