I am a 35 year old educated man and keep asking myself how I got in this mess and why I stayed for so long?
You are lucky to be an educated man but you must analyze if you had previous knowledge of her BPD condition, if not it is not your fault, she should have been honest and told you about it. You are not responsible for her lack of honesty.
Why did I stay for so long?
Once we realize what we got ourselves into then we notice how much time we have lost and the kind of hell we are living. Be grateful you are not married and that apparently you have no children with her, thus you can leave and start anew without guilt.
Is it normal and natural to doubt yourself on why the relationship ended with a BPD person?
Yes it is normal, BPDs are very intelligent and manipulative they can convince who ever they wish on what ever subjects are convenient for them. DonÂ´t let it get to you, be a bit selfish and protect yourself.
I wonder if I have heard that its all my fault so much that I started to believe it?
Yes, it is called brain washing. That is what BPD persons do to who ever they wish to hurt and/or control, have been there! But again don't let it get to you, analyze what has been happening and do your utmost to have peace and be positive about your future with someone who is not BPD.
Good luck and God bless!