@ConstantlyFireFighting.
Firstly, I am sorry that you had to go through this, as someone with first hand experience, of CSA by 2 different people at different stages of my life, I completely understand how this makes you feel.
Secondly, being able to open up to your Mother is a really big deal.
OK so the first one Mum found out whilst it was happening and was dealt with. the second, happened when I was 6yrs - 10yrs, I did not tell Mum until I was 21!
My Mother and I, never had the best relationship at all, so it was really hard to get the courage to tell her, as without her support, I was never going to be able to bring him to justice.
fortunately, she was very supportive and whilst it may have come as a shock, she never ever doubted anything, nor told me to forget about it. In fact, when I told her that I had gone to the police, her words were simply, "good".
I am sorry that you Mother has reacted in this way, I do not know who that person was in your life whist he was SA you, but it could be her way of trying to not become attached to the situation, due to now finding out and knowing that she was not there at the time to protect you.
If you want to try again with your Mother, perhaps you can write her a letter, telling her what happened, (not exactly what happened, but the outlines), how the past made you feel and if you are intending on doing something about it now and tell her that you would like her support, as she is your mother and remind her (dependant on who this person was) that she is not to blame.
SA affects people in different ways and neither are predictable.
I hope that this made sense and I really hop that you can find peace with your Mother as well as get some closure on the terrible experience that you had to endure as a child.
x