My main question is why is it I'm so quick to be defensive

Thank you very much for your question. I know that it is not easy to ask for help or challenge our behaviour, and that's why I truly appreciate the effort that you are doing by asking this question. 

Let me also say that I completely understand what you mean, it happens to all of us that, sometimes, we take things as an attack and we get a bit defensive, it's completely normal. We do feel angry because of this and react to that anger, with the outcome that you are describing so well: we try to prove our point even knowing that we are wrong.

The main reason that is behind this situation, from my honest point of view, is the fact that we try to be perfect and don't allow ourselves to be wrong. We tend to feel like, if we fail, our knowledge has been challenged and we "need" to be right.

Why is this happening? Normally, because we are too judgmental with ourselves and don't treat ourselves as good as we should, as good as we treat others... Because of this, our self-esteem is not as high as it could be, and we don't like ourselves as much as we should. When this is happening, we don't have the best of confidence in ourselves either, as we feel like we are not that good. Finally, also related to this, we criticise ourselves a lot and we are so hard when we assess what has happened that everything looks like a mistake. 

In this sense, I would recommend trying to improve your self-esteem, mainly by talking to yourself in a kind and not judgmental way, and also acknowledging all the little and big things that you do well in your day-to-day. Another important point is to start communicating in an assertive way, so you respect your point of view while respecting the others' points of view, making an improvement in a lot of contexts of your life. One other thing is to learn how to deal with our emotions in a more useful way. Finally, speaking with a professional about your issues to feel listened to and understood is also something that can be very useful.