Just wanted to add, i've been in a similar situation for nearly two years now, and was so emotionally and verbally abused i developed an eating disorder, was so skinny even size 8 jeans wouldn't stay up!
Now i'm in a much better position, so much stronger but still have down times. Last week my ex collected our two youngest sons on Sunday, and for the first time his gf was in the car, usually she waits at her home, what it did to me, waving my sons off with her in the car, was as bad as it was in the first few months.
But now nearly a week later, i've felt strong enough to tell myself off for letting it get to me so much. So what if shes in the car, i've done nothing wrong and got nothing to get upset about, shes the one who started a relationship with my ex.
My eldest son died almost five years ago, bereavement to me is so similar to grief over the end of a marriage, it comes and goes, you're not regressing, after more time has passed you'll be able to look back and see the worst times get further apart, and the nice times grow. Think as the worst times get less, you notice them more. Its a shock when they happen again, so don't think you're regressing, you're not you're moving on all the time even when it doesn't feel like it.
Sending hugs, wouldn't wish this on anyone. x