You had nothing to do with the miscarriage. If stress alone was enough to cause miscarriages, the human race would likely cease to exist. There is always stress in life, and sometimes that stress can be overwhelming at times. That stress does not magically vanish when you become pregnant. In fact, the stress can increase. Don't get me wrong, it is probably better for both mother and unborn child if the mother is calm and relaxed, but stress does not cause miscarriages. Their are women whose husbands/partners die during their pregnancies. Talk about stress, that would be one of the most stressful things an expectant mother could endure. But the overwhelming, vast majority give birth to healthy, full-term children.
You are right, your girlfriend is extremely insecure, but this is HER problem, not yours. There was absolutely nothing you could have done to rid her of the insecurity. That is something deeply rooted in her that only she has the power to resolve. Unless she is willing to acknowledge that she has a problem and is willing work toward resolving that problem, her insecurity will never go away, no matter who she is in a relationship with. You do not have the tools, resources, and expertise to help her with her with those kinds of things. That is something only a professional counselor or mental health professional can help her with, but even then, she herself must first and foremost be willing to put forth the work and effort into overcoming these kinds of personal problems, or even the most talented, experienced experts in the world cannot help her.
Insecure people have a very high tendency to make their problems their partner's problems, and that is very unfair and unreasonable. I can almost guarantee you this, if the relationship had not ended because of issues from your ex, it would have eventually ended because of another reason. Insecurity is one of the most toxic things in a relationship, and she most certainly would have found another issue that made her insecure, and would have projected that onto you. The relationship was doomed from the beginning. If one is not secure, confident, and in a healthy state of mind, there is no possible way they can expect to find happiness in any relationship. Again, there is something in her that isn't right, and only she can take the steps to resolve that issue.
As far as the failed pregnancies, it is completely normal for both the mother and the father to go through the grieving process after the loss.Take the time you need to grieve and to heal, and then you must be able to move on, because there was nothing you could have done that would have made a difference. Miscarriages are sad and tragic, but unfortunately, they are a part of life. Roughly 1 in 4 women experience miscarriage. Again, it is sad, but not uncommon.
Your girlfriend seems to have a lot of miscarriages. Either there is something physiologically wrong or the odds are astronomically stacked against her and she is just experiencing an unfathomable bout of bad luck. Personally, I think it is suspicious considering her behavior. Please do not take offense, as I in no way want to minimize your loss, but are you completely certain that she was actually pregnant every single time she claimed to be? Was the only evidence of the pregnancy what came out of her mouth? I am only asking, because women who are insecure, possessive, and controlling the way you described have been known to fake pregnancies and miscarriages in order to gain attention and sympathy from their partners and in order to gain more control over them. If she can blame you for the miscarriage, and if she can make you believe that you were responsible, then she can make you feel horrible and guilty. For people like her, guilt is their best tool. Once she can make you feel guilty, then the level of control goes up even higher. I am not saying that she was never pregnant. I'm just saying her behavior regarding the pregnancies and miscarriages is suspicious.
You cannot blame yourself. Even if she in fact did suffer multiple miscarriages, it had nothing to do with you. In fact, if she truly was pregnant that many times and lost each pregnancy, that certainly increases the possibility that there was something medically wrong with her that prevented her from carrying to term. But regardless of what the factors were that led to the miscarriage, stress was not one of them. Grieve for your loss, take time to heal, and pick yourself back up and move on. Do not carry around the guilt, because it wasn't your fault.