It is unfortunate that your girlfriend can't compromise on this issue. I would try changing certain things so that she doesn't feel that you are focused on this issue. First be conscious of the way you ask her for oral sex. If you are perceived by her as deserving it for things you have done then she may be less open to fulfilling your wish. You want her to feel that it is her choice so that she doesn't feel obligated. Women don't tend to do well when they obligated for something. Be conscious of also comparing yourself to ex's. That can be a slippery slope. I would also find out if you can why she doesn't like to do this. She may really hate performing this act. If that is the case then asking her may make her feel that you are asking her to do something unpleasant. We don't know why she has changed her perception about this act. Lastly be patience. She could drastically change her mind over time.
Please press accept; this is the only way I am compensated