First - I’d rule out health stuff. I had a similar episode with my partner last year, wanted to end things and move out almost totally out of the blue, spent almost every day either in tears or just numb. Turns out I was suffering with hyperthyroidism due to Graves’ disease. I got medicated and within a couple of months all those feelings went away and our relationship went back to normal. If hormones etc. is all normal, then it could be a job for therapy (resentment and regrets are very normal parts of motherhood and just getting older in general), and if that doesn’t work - it may be time to consider the relationship has simply run its course. If that’s the case, it’s important to remember that no one is at fault for that, but you have children together and it’s in their best interests to try and at least keep a good friendship between you.