Shame and power

This is a difficult and complicated question to answer because you are asking about the two core childhood wounds of FA: having a chaotic upbringing in which the child believed they weren't good enough due to the lack of attunement from the adult parent figure, leading to shame, and a fear of engulfment developed out of the source of everything a kid needs to live plus their comfort also being a source of fear, pain, and unmet needs (come here, go away). That is their experience with receiving love, and FA was a childhood defense mechanism to stay attached to the adults they needed to physically survive but who caused all this cognitive dissonance in spite of all the pain. So there's nothing you can do about this because the issues are all his and unrelated to you. It can only improve if he decides he is tired of living in so much mental pain and anguish and wants to change and get help to start to heal, usually professional help of some sort. You should continue focusing on coparenting and what you need for yourself to be around him without it causing you anguish (for example, if you still have feelings for him and a fantasy that you can get back together and something will magically change if you do so you'll be happy together, then it will continue to be excruciating to be around him).

You can search the site for the words "shame" or "fear of engulfment" for more threads and conversation that the concepts are embedded within. Or just keep reading fearful avoidant threads, maybe within the fearful avoidant support category. Those threads are written by FAs who are sharing their experience and trying to do work on themselves and asking for help (not for people to ask them questions about FA partners). You should also ask yourself more questions about your own attachment style and what keeps you trying to reunite with an emotionally unavailable partner who can't meet your needs. Yes, I know you have a child together and life is theoretically simpler if parents can stay together, but it isn't always healthy or even good for the child when the relationship between the parents has an insecure dynamic. The child observes and learns that's normal and okay, and will emulate it later on. So if someone is unready or unwilling to seek help, separate households with good, and consistent coparenting between exes who respect each other can actually be healthier.

Okay, that's good he's seeking help! Just remember that having your own needs and healthy boundaries is not a lack of understanding just because someone is avoidant. Exploring your own attachment style and issues is good, as if you're anxiously insecure and with an avoidant person, there is a lot of textbook clashing that happens. But take accountability for only as much as you're actually responsible for, and don't lean too far and take on too much responsibility for the relationship in an attempt to change his behavior. That's dipping into codependency and enabling dysfunctional behavior territory, and an easy place to get to if you're anxious and over-correcting for issues that are actually his.

Acceptance goes a long way for me. Letting me be me lowers the shame. I don't want to feel like I'm broken or my partner is tolerating or having to change for me. I also have a Spidey sense for this.