For a very long time I walked through the world with an armor such as you speak of. As jana said, not even those closest to me knew some of the deeply important (to me) aspects of myself.
When I was about 35 I had a paradigm shift that opened me. Many significant changes happened and for the first time in a long time I allowed true love to enter into my heart - both for another - and myself. When this happened - my armor became more flexible and penetrable. I think I was more strong in my Ego, my centered space, because of the love of my other, and the fact that I allowed myself to let go of Duty and seek my own reasons for being happy.
Yet I still kept the armor - even between me and my beloved other. (shaking head no - this was a mistake)
About 4 years ago I attended a retreat/event of sorts over a period of 2 weekends. The event was designed to help a person identify their shame/guilt/hatreds/fears and then teach the person how to let them go and obtain peace of mind. The event was transformative for me in that I once again had a paradigm shift and this ultimately led to a letting go of the armor.
I found that the less armor I had around my heart - the more I was able to accept people and myself as I am. Ironically this vulnerability to being hurt and pain has allowed me to feel more loved than I've ever experienced before. These days it seems the more I give it - the more I get it.... Love that is.
The retreat event taught me something very important about people. We all go through the same processes of wanting to be loved, being scared, being humiliated, being happy, and so on. No matter where we come from, no matter what we believe - or - not believe. While we are unique - we are also the same. I know we are all connected. For a long time I knew that from a physics perspective - but now I know it from a universal truth perspective.
So now when I'm around people I don't know very well, I watch and I listen to see if there is a connection I can sense between us. If there is one - I'll speak whatever is on my mind and see how it plays out. Many times people never ever get me. Frankly, that makes me feel lonely - but I can't expect them to understand me - for most do not view their world as I do. Their words cannot really hurt me - unless I let them - by having some expectation or attachment to the relationship.
Since practicing meditation and mindfulness, I don't have this problem as much anymore. I just try to love the person in front of me for who they are and the fact they are trying to be the best person they can be at the time. IMO most people are trying to do the right thing within their limitations of education/environment/economic situations/health and so on. Of all the types, the INFJ is best at understanding and showing compassion for what humans have to go through these days.
Praefect. I can only speculate why words can slash your heart and make you bleed with pain. I might wonder if your pain is already there from some prior life experiences and when those words are spoken - the pain is triggered once again. In my experience, the only way to let that pain move from your heart and release the energy that's bound up in it - is to share it with others. The caution here - tho - is to do this with a safe person(s). For that to happen - you have to open a small window in your armor and let something out - as a test - and then see the reaction from the individual in front of you. This way you learn how to feel if there are connections between you and others. Another positive of this action, is you've opened your armor up and faced your fear. Like SouloftheLaurel says - tell yourself this doesn't bother me - and then take a little risk. If the person in front of you totally 'effs it up' - then say to yourself - well that one is off the list - feel your disappointment - show compassion for yourself and your loneliness - and go on to meet another.
Hope this helped you in some small way.