Boyfriend won’t say no to his ex

I’m in a wonderful relationship with a man who has 2 little girls from a prior marriage. I’ve noticed he hates to say no to his ex when she asks him to take the kids last minute which results in him canceling on me. I’ve been flexible and easy going so far but it happened twice this weekend and it’s bothering me.

C L canceled our plans tonight
T He’s putting his ex before me.
F disappointed
A I start to panic that he will always allow her to dictate his time, freak out, plan a conversation with him, second guess my commitment to him
R I’m not putting myself first

C L canceled
T He’s taking care of his kids
F At peace
A Enjoy my evening without him, discuss this with him calmly, make plans to have the girls come to my place
R I put myself first

I like my IM quite a lot and it has calmed me down but then I start to panic again about what it means for me that he won’t tell her no. Do I need to run a different model?