Girlfriend won't get over MY ex

Hi people thank you for reading my post. It might be quite long so I apologise for the long read.

So my girlfriend is very caught up on my past. The fact is that I've only had 1 girlfriend prior to her. However I am her first boyfriend. She is very 'immature' emotionally in that she wanted her to be my first (Although I guess many people want that).

I really love this girl. However I did not love my first girlfriend, I went into that relationship impulsively. I also lost my virginity to my current girlfriend. The reason she is so caught up on my past is because I have rejected her twice. The first time she confessed her feelings I was still with my ex and the second time I was single, but I still rejected her but afterwards I really started to like her so I told her. She is very brave to have confessed to me twice. She always asks about my ex like what we did etc, even though she knows it will make her unhappy, sad and angry. I believe that she does this because she thinks that "what was so good about your ex to have rejected me".

Her insecurity is certainly justifiable. If I was rejected for someone else, I would also be insecure. She tells me that she thinks about me and my ex involuntarily and its makes her really sad. I have told her that I love her... I express my feelings constantly trying to make her feel better but it doesn't to be working.

She understand that she shouldn't be this way. She tells me that it's unfair for me to deal with it. Logically she knows but she can't control the feelings of insecurity and sadness. She doesn't want to think about it but she does.

How do I help her move past my ex. I never mention my ex... is there something I can do? I really love her... I've never met someone so caring and so loving.

Thank you people (PS we're both 19)

Refuse to answer any more questions she asks about your ex.
When she brings her up again you tell her that you guys will never be talking about her (the ex) again if she (your gf) brings her up then you're leaving her company until she can stop the OCD thinking on someone you don't even give two fks about. You tell her one more time that it's over with the ex, you're with her now and the two of you are only looking forward now, you love her and you can't do ANYTHING to change your past so she, from this day forward is NOT to bring the ex up again.

Then you grab her by the hand and you take her out to do something new that neither of you have done before and have a good time... just you and her.

give her one big talk tell her to ask you every thing she wants to know about your ex and you answer all her questions and tell her never to ask or talk about it any more

Yeah you can't talk to her about it whenever she asks or it will never end. One last convo, something along the lines of what wakeup said, announcing that you love only her, you don't care about the ex, but that love means trust you so you will not be discussing this anymore. She does sound immature. This sort of thing can really ruin a relationship.