Discussing donor conception is not just difficult for parents wishing to disclose or even discuss it after disclosure. For us offspring it is actually even more difficult, as there is a sense of respect and need to not hurt our parents feelings. It can be very hard to bring up the topic for a variety of reasons. Almost all offspring have at one time or another felt that we cannot talk to our parents about our biological father. Joanna Rose discusses the idea of "existential debt", where as donor conceived offspring we feel that we are indebted to our parents for having us in this manner and therefore must be more concerned about their feelings than our own. They do not want to hurt their parents, because they have been taught that their parents infertility is more important than their right to know both biological parents.
I began searching for my biological father almost a year before my mother ever learned of what I was doing. It was the spring of 2003, and I was a senior in high school. I accidentally chanced upon an episode of Oprah that was talking about donor conception. That evening I went on the computer and began investigating. I found the group PCVAI, and there I was able to talk to others like me, and finally overcome some of my fears. I would strongly advise you to join PCVAI. It may help you to learn how to talk to your mom.
Over time I gained more and more confidence and eventually was able to talk to her more, but it was all because I met others who gave advice and knew where I was coming from. While I must admit, it's not something that we are even comfortable now talking about, at least I can talk to her when it is necessary - such as to get her DNA for a siblingship test. And I've come to terms with the fact that she may never be happy with my views on a decision that she made 25 years ago.
If you see this, please send me an email, because I would really like to help you as best I can.