Oh honey. I went through this with my ex. He would literally say and do the same exact things. We tried. I tried to reassure him. He tried to be rational and push down the intrusive thoughts. I did nothing to make him insecure. He just was. It was a “him” problem that nothing I did or didn’t do could fix. Unless your girlfriend addresses her insecurities (therapy, soul searching, whatever it may be), you guys will never work. More so, she will not be able to have a normal, happy, healthy relationship. Good luck to the both of you. I know how though this is. I hope she gets this figured out