Time, give it time. All things
intimately sexual take time to develop. Everyone knows how to fuck, just stick the right thing in the right place. But to really gain trust and skill... takes serious investment of time and energy. Open yourself to being vulnerable with her and telling her about your past, and then I am almost positive that she will be more receptive to how you feel about bj's.
It took me months and months to learn how to give consistently good head (I think?--from the cumming results as my data points)
... and that process went hand in hand with learning to enjoy the experimenting, and from working against any barriers in my own mind. Likewise, it took me a long time to be able to receive head, and even longer to learn to cum. It is a process of becoming more and more vulnerable and trusting... try to enjoy that time, and let us know how it goes.