Hi OP I'm a step-parent. I had similar emotions for the first few years. I worked through it, and am not proud of how badly I dealt with it at first.
I'm a bit of a rigid thinker and just wanted a different sort of life for myself, and had to compromise so much more for so much less. Looking back, I wouldn't do it again, but I love DP and my SC. Life has worked out differently than I hoped but it's okay, am glad I learned to overcome jealousy, and to have learned a lot about myself.
A few suggestions, in no particular order:
How much do you love him - can you make dealing with this part of your commitment to him - he can't find it easy being tied to someone else he finds difficult.
For me, having my own child put a lot into perspective. Can you think back to what it was like with a young child?
The mother may have great strengths and be a great mother but not be particularly great at housework/being organised in the morning.
The child won't need dressing & ferrying to school for much longer.
Thank your lucky stars the boy is healthy and able - things could be much tougher and sadder for you all.
You will need your partner's forbearance at some point, you don't know when. Right now he needs your tolerance; at some point you will need his.
Do you want to be the kind of person who polices his phone calls?
No partner is perfect. This one comes with this baggage. Others have other baggage, even if they don't have children and a crazy ex. And remember that all men with children say their ex is crazy. Others are workaholics or have substance issues, or indebted, or they bang on about their hobbies incessantly, or can't relate well to other people, or are extremely needy/controlling/whatever. If he's generally sound apart from this - and him being a good parent must make him a more attractive person - think realistically about whether the dating pool offers much better. And if you find day-to-day compromise/fuzziness difficult, just be single and don't look for committed relationships.
Try therapy. If you can't access affordable therapy, stints of working through ChatGPT can really help. I spent a morning recently when processing some resentment and by the end I had worked through it.
Try and visualise yourself being friendly, if not friends, with the mother. Think of the best possible version of her. I gradually learned that my SC's mother was funny and caring, and always put her children first. Think about what you have in common, and how you're ultimately on the same side - you want the child to grow up well and launch into adulthood successfully. Nobody is a unicorn, and there will be some things that she is very capable at that you struggle with.
Things will also change when she finds a new partner - she will look for emotional support less - though it may bring other complications, e.g. if her partner has children and a difficult ex.
Don't let your resentment show to mutual friends or your partner's family - it will reflect badly on you, even if that feels unfair.
When your partner's busy with his child or doing school runs or whatever, that gives you time and space for other things. Don't put all your focus on your partner & family life. Work, friendship and outside interests are really, really important for women in their 40s.
Life is long - then suddenly short. You can do this. But if you don't really want to, that's okay. Bow out gracefully and work out what you want out of life and relationships.