Hello John!
Yes, this is normal behavior for any normal person to save the dear man (or woman) from any sad and bad things.. And this is normal your girl-friend wanted you do not know about the unlucky experiences in her PAST life..
So, you know about now . And what?? - You are unhappy and your behaviour is strange and unbearable, and you want to stop this relationships only because you are
jealous very much? You can invent any reason why you do broke up with this woman after being in a relationship for 18 months. But the main reason is that you do never love her..
I wonder if you had not any relationships in your life before you met this woman?? She had the same..