OP, I feel for you, because when deep, old insecurities are re-triggered, it doesn’t matter whether you’re being reasonable or not…the pain is the same. You FEEL that you will be doomed to last place with this DP, that whenever his Ex turns up and needs something, you will be dropped like a hot potato, your needs unmet, your existence unimportant, unwanted even. If your dad wasn’t around when you were growing up, I can see how this might bring back those very sad feelings and make you expect that outcome.
You feel this even for reasonable situations, like them attending a school function together as friendly co-parents (not as an intact family pretending you don’t exist…or do they behave like a couple? Is your DP trying to make up for his past poor parenting by going back in time and re-creating a happy intact family with his Ex for an evening? What does his behavior suggest? Do you feel the Ex is wanting to play happy family too, what is her behavior like? Or is she simply amicable and appropriate? All of these factor in to whether you will be able to deal with the situation or not.)
There are some things that I know I could never be comfortable with in a relationship, because of my own upbringing - even if they were innocent and appropriate. They would simply be too triggering for me. I’ve worked hard to understand these limitations and respect them.
It feels horribly unfair when you see an important person in your life treat someone one way - treat them well - and you think, “ok, if and/or when I’m in that same situation, that’s how I will be treated, too, because that is fair and I am equally deserving, too.” Then, when the day comes, you find yourself treated totally differently. The rules have changed, or at least don’t apply to you.
I imagine, now that you are faced with the prospect of being the mother of his child, you have a very strong fear that you won’t receive the same consideration and loyalty that he is now giving to the mother of his existing child. In other words, that you and your child may have that role, but not get to occupy it as fully, in fact finding someone else already in that spot taking priority and his loyalty. I’m talking about how it FEELS to you. Regardless of what is right, co-parenting wise, you don’t want to spend your life FEELING anxious and angry. It may be that this is too hard for you, even if his co-parenting with the Ex is appropriate. (And if it’s not - if they are still enmeshed in other, less appropriate ways - you need to decide if this is something you want to spend your efforts fighting).
I don’t have an answer, but trying to understand the origin of your feelings is a first step. So is figuring out whether your DP has any sensitivity to your fears. If he’s not excited you are pregnant, that’s not a good sign. You don’t deserve to get the leftovers of someone’s emotions. Perhaps he will be more excited to look forward about it with you, after this very significant period of his older kid starting school has calmed down a bit. Maybe it’s just a very, very focused, reflective time for him that he particularly wants to get right. But that means waiting to see, and I imagine that will be hard for you.
Please find support with friends and extended family right now, disengage as best you can from the co-parenting situation, and look after yourself while you work to understand your feelings. Good luck.