I assume there are reasons you are afraid to tell her, and I'm genuinely sorry that's the case, but you do need to tell her and maybe it won't be as bad as you think.
When you do tell her, be clear about what it means (divorce, you intend to date women, where the children will live).
My ex husband let his brother tell their dad in the end and ex-FIL didn't want to believe it. I ended up in an extremely distressing meeting with ex-FIL, at his house, where I had to tell him that yes, his son was definitely gay and no, there had been no full and frank discussion before we got married (XH had somehow let him think I had willingly and knowingly married a gay man). And yes, we were definitely getting divorced because, no, I wasn't going to wait for him to 'come to his senses'. Made an already traumatic separation and general situation even worse because my ex still didn't have the guts to be honest. We'd been separated six months at this point.
For what it's worth, after a bumpy start, ex-FIL appears to have fully supported my ex, despite XH treating me appalling, and has embraced ex's boyfriend, who now joins them for family occasions. Your mum will probably support you because she's your mum. Don't blame your ex (when telling your mum) for the situation, and don't let your mum blame your ex either.
If your ex needs support, he can contact Straight Partners Anonymous.
I also hope you're being considerate of your ex. Most of the time, the newly out gay person seems to move on at lightening speed, leaving their devastated straight partner to pick up the pieces - and it's shit. Please don't be that person.
I'm sorry if that sounds harsh, but I'm speaking from experience. I'm not on good terms with my XH but that's not because he's gay, it's because of how badly he handled his coming out and leaving me. It is always better for the children if the separating parents can be respectful and considerate of one another.
Good luck. And, if nothing else, remember that if your mum can't be supportive then maybe you're better off without her in your life (much).