I'm not in a situation like you, but maybe my tips will help anyway ....
Since you are not worried that your children might not be treated right at their mums, as you say she's a good mum, try to focus on that. Tell youself time and time again that while it's okay for you to be sad, your children are being well cared for and happy while with their mum. Keep reminding yourself what a great thing it is for your children to have both parents actively involved in their lifes and to be able to spend an equal amount of time with both of them.
And it's also worth to keep reminding yourself that being sad is not necessarily a bad thing - both for you and your children. Once when I settled in my daughter in a new pre-school, the teacher told her it's time for her to go home and see you tomorrow. My daughter wasn't happy about it, because she enjoyed herself. The teacher then said something, that I found both lovely, helpful und very true - and I keep bringing her sentence up (in my head just for myself or aloud towards my kids), if it fits:
You should leave, while you are enjoying to ge here, because then you'll look forward to coming back.
Maybe both you and your children can concentrate on that - that it's good to be sad about leaving, because it means you are having a nice time together. And you can all look forward to when they are coming back. And your kids can look forward to being with their mum.
You can also try to keep busy while they are with their mum. If you have some flexibilty in your job, you can work more during those times and less when they are with you. Or you can schedule things that need to be done for those times. Or make plans to look forward to - going to the gym, out for a drink with a friend etc.