I initially thought your OP might actually be a reverse, as it seemed so unreasonable in terms of your expectations OP.
Speaking from my own perspective having been a stepmum in a relationship with a man who was unable to draw appropriate boundaries with his EXDW (he is now my EXDP, but we share DC) you need to find a way to accept the new way your EX wants the relationship between the two of you to change.
It sounds like your EX is more than willing to, and continuing to, have a relationship with his DC, as it should be.
I hate that my kids miss out with having both parents at certain times in their life if I'm honest.
The above quote from your post is a by-product of DC having parents who are no longer in a relationship with each other, I'm afraid, and you do need to find a way to manage these emotions, given your EX withdrawing from you (not your DC).
Your EX really doesn't need to be there for your DC's bedroom decoration, however nothing in your posts indicates that your EX has an issue with attending things like your DC graduation etc alongside you, which is the important thing to focus on.