It's easy to get carried away by your emotional responses (frustration, impatience, feelings of failure and inadequacy etc) and totally forget that you are experiencing increasing measurements! WHOOP! ;D
BTW, if you're a slow responder, then what on earth does that make me?!! Haha! But I choose not to use any unhelpful labels and instead repeat out loud "I am growing full B cup breasts" in my daily affirmations. (Yes, I actually do this every morning in front of the mirror. I figure it can't hurt and is certainly better than berating myself).
We don't realise the power of those two little words "I am", but anything that follows them (a slow responder/flat-chested/unsexy...) either becomes our reality or continues to perpetuate it. You're experiencing definite, measurable increases in size - don't sabotage that! Focus on the positive, even if sometimes it feels like you're faking it. The brain actually doesn't know the difference!
But seriously, your post is a useful reminder of the importance of self-awareness. You've already acknowledged that the PM makes you feel emotional - so next time you feel low, just step away from it and say something positive, encouraging and supportive. Be your own Boob Coach!
I realise this might sound like a bunch of pseudo-psychological nonsense (I've had a lot of life coaching, therapy and healing in my time, so I know the lingo!), but I honestly believe it's better than the alternative - feeling hopeless, miserable and self-defeating. I often struggle with cynicism and my logical brain picks holes in everything, so to counteract that I choose to shine positivity on everything! It does help.