When we say that you are being "invalidating" to your partner, we aren't saying that you are being intentionally mean or cruel to them. Most of the time we are just trying to explain ourselves or correct a misunderstanding and it comes across as invalidating to the pwBPD.
Some examples of invalidating responses:"I didn't mean it that way!"
"... but that's not..."
"This is what happened"
"NO, you've go it wrong!"
"I AM doing it right"
"you're not doing that right. Let me do it for you"
"I was only trying to..."
"Why can't you just let it go?"
"why do you always have to do this?"
or it may just be the way we sigh, raise our eyebrow, or even worse - roll our eyes 
Something as inconsequential as adjusting the temperature can be seen as invalidating someone's feelings. If the pwBPD is cold and turns up the heat in the house, then you come home and turn it down, your actions are essentially saying "
they shouldn't be feeling cold"... when in reality they "are" cold.
Invalidation often comes up when there has been a misunderstanding between two people. Both people believe they are right and the other person is wrong. Both people work to get the other person to change their minds and come to an agreement with them. How often have you been in a conversation where the more you tried to explain something, the less the other person seemed to hear you and the angrier they got? When we try to justify ourselves, or explain or defend ourselves, we are telling the other person "they are wrong" and invalidating their beliefs.
You may very well be right, but when dealing with a person who is extremely sensitive, quick to react, always looking for criticism, and easily dysregulated, your words come across as invalidating
to them.Feelings and emotions can never be wrong. They are based on our beliefs and our interpretations of things, so telling someone that you don't agree with what they are saying means their
feelings are wrong. How can a feeling be wrong?
Telling a person she shouldn't feel the way she does feel is akin to telling water it shouldn't be wet, grass it shouldn't be green, or rocks they shouldn't be hard. Each person's feelings are real. Whether we like or understand someone's feelings, they are still real. Rejecting feelings is rejecting reality; it is to fight nature and may be called a crime against nature, "psychological murder", or "soul murder." Considering that trying to fight feelings, rather than accept them, is trying to fight all of nature, you can see why it is so frustrating, draining and futile.
We need to accept that a pwBPD doesn’t think the same way we do, and that while many of the arguments seem to come from nowhere, or make little sense, they are often triggered by us when we inadvertently use an invalidating response. A pwBPD and a non speak different languages. When the person with bpd is triggered, they express themselves from a position of pure emotions and primitive defenses. There is no logic to what they are feeling, which is why they have trouble articulating and expressing themselves. Often, even they don’t know why or where the feelings are coming from.
We, on the other hand, try to approach the situation from a logical standpoint. We believe that if we can just find the right words or phrases, that our argument/words will suddenly make sense to the BP and the fight will end. We might as well be speaking Polish to them though, since they are in an emotional state and we are defending with logic. See the problem here? Our own defenses make things worse, since we use terms and defenses that invalidate how the BP "feels".
The more we try to explain and defend ourselves, the worse things actually become. We throw fuel on the fire by invalidating their opinions, beliefs, statements, ideas, suggestions, or emotions.
They are emotional - we are logical. Two different attempts to communicate.There are also times when the pwBPD may not be accurately expressing themselves, causing us to respond to what we heard, not what they actually feel. So when asked "how are you?" they mumble "OK" instead of their true feelings of sadness or fear. Then later on when they explode and accuse us of not caring how they feel we are left confused - but we asked and you said you were OK!" ?
It's not possible to prevent "all" instances of invalidation.
To help stop things from getting worse we need to begin by recognizing what
we are doing wrong. Before we can
validate, we must stop
invalidating.
Words and phrases that are invalidating to others (not just those with bpd): *ordering them to feel differently-
"Don't be mad. Get over it."
*ordering them to look differently-
"don't look so sad."*denying their perception or defending -
"that's not what I meant"
*making them feel guilty-
"I tried to help you"*trying to isolate them-
"you are the only one who feels that way"
*minimizing their feeling-
"you must be kidding"*using reason-
"you are not being rational"
*debating-
"I don't always do that"
*judging and labeling them-
"you're too sensitive"
*turning things around-
"you're making a big deal out of nothing"
*trying to get them to question themselves-
"why can't you just get over it?"
*telling them how they should feel-
"you should be happy"
*defending the other person-
"she didn't mean it that way"
*negating, denial, and confusion-
"now you know that isn't true"*sarcasm and mocking-
"you poor baby"*laying guilt trips-
"don't you ever think of anyone else?"
*philosophizing and cliches-
"time heals all wounds"*talking about them when they can hear it-
"you can't say anything to her"*showing intolerance-
"I am sick of hearing about it"*trying to control how long someone feels about something-
"you should be over that by now"*explanation-
"maybe it's because _______ "
Actually, go through each of those invalidating statements/responses and think about how it feels/how it felt when someone (not necessarily the disordered person in your life) used those statements on
you!
Even if you aren't part of the problem - you
can be part of the solution

To see the whole web site for deeper understanding:
http://eqi.org/invalid.htm#Examples%20of%20invalidating%20expressions.