Haven’t rtft…
But, on top of your ex being the children’s mother which naturally predisposes them to her, I would say there is still a sense of novelty in the new arrangements.
Previously, you were stay at home dad and mother wasn’t with them much. Now she is responsible 50% of the time, she will be showering them with experiences and gifts, the new partner too as he can afford to, and it’s all adrenaline, E numbers and endorphins.
It will settle down, if she was self interested previously she will revert to type.
If you have mental health issues and decided to be in low paying career, perhaps to facilitate the children’s early life then, because they are children and don’t understand health issues and career choices, there might be a predisposition in them to say it’s dad’s fault for behaving in X way and not having a high flying career to keep Mum.
I know that’s not nice to think about, but they are children and it’s all very black and white. The relationship between the mother and this man is all very new and still in the honeymoon phase.
Things will not be shiny and new forever, wait until it all calms down and normalises.
Until them keep being the constant, stable father you have been, don’t bad mouth either of them, and use the time you don’t have them to focus on your own personal development and what makes you happy.