What an amazing step, Poet. For what it's worth, there are dozens – hundreds? – of women here who are so proud of the growth you've achieved, and the strength you're showing. Well done!
If you can write something out for your mum, to give her the general facts, that might be the way to go – you can take your time, pick your disclosures carefully, and don't have to tell her face to face, which might also help her have time to compose herself, digest the information, and react helpfully.
As we say in NZ, kia kaha! Stay strong!
The next few weeks will be a roller coaster of relief and grief, anger at him and worry about him, being glad to be away in peace and freedom, and lonely and frightened about your future. So when you have a quiet moment, write down what you know – what he's done to you in terms of rapes and control, how he treats you and the kids, how unhappy, tense, and scared you were, etc, so you have that reminder of the facts when your feelings get confusing.