Is “he became better for someone else” a real thing?

Out of curiosity, how would you feel if someone was trying to change you? Would you become "better" for them? Or would you be resentful unless you were ready and wanted those changes for yourself regardless of the other person?

I, for one, don't change for other people. Sure, when I was AP I'd try to pretzel myself to accommodate, which made everyone miserable. But I'm open to new ideas and interested in self-improvement. So I don't actually change FOR anyone, but I always listen for better perspectives and ways of doing things (more efficient, more interesting, etc).

If someone did change "for" someone else, then they sound doomed to revert back if that person is one day no longer in their life. Pretty codependent-sounding mindset, to me.

So....just because someone dates a long time or gets married doesn’t mean that he or she has suddenly done the work and become secure.  Sometimes...and i would dare say..the majority of the time....it is simply a matter of being with someone who tolerates “the dance” more or..someone who triggers the avoidant into an anxious state.  I think the notion of someone becoming secure for a relationship is rare.

One thing that was helpful for me was reading through every FA thread on this forum when I first found it and had these same questions. (And yes, even though you have your ex pegged as DA, he sounds FA to me to stay in the dance so long in addition to his earlier life circumstances and coping mechanisms, but you see the avoidant side rather than the anxious side because him pushing you anxious triggered him avoidant and not usually anxious.) Every single story followed the same trajectory. Even if the timelines were different and the details slightly different, pairing dynamics that people wrote about with an FA partner / ex who wasn't nearly as invested in changing as the OP of the thread were the same in every single post on this entire forum.

That really convinced me, after years of waiting on potential, that believing everything could be perfect if an avoidant (whether DA or FA) partner could just change is a fallacy.

What it comes down to is, it's a misunderstanding of attachment theory. It's very, very difficult to understand the mindset of a different attachment style than yours if you've never experienced it. I viscerally get AP, I get secure, and I get the anxious-leaning side of FA because I've actually experienced those mindsets. I don't really viscerally understand being triggered avoidant because I don't have that nervous system wiring because I haven't been in the environment during formidable years that would trigger it, though I rationally understand it and so can empathize and respond appropriately. But I know I can imagine the pain healing AP because I've been through it, and I can't imagine the pain in avoidance and how tightly someone may cling to kicking the can down the road forever because I personally don't cope that way.

I feel like what you're actually clinging to is what you said about your dad. Your dad eventually decided to change. You're lucky! Most people don't make that decision (I've cited older studies that say about 25% of adults change attachment styles in their lifetimes, and I haven't seen a breakout of if that's anxious earning secure, avoidant earning secure, or secure becoming insecure). Your dad also didn't make that decision until he was in his 60s. If he's still alive, did you ever ask what prompted it? Everyone I've ever spoken to who has earned secure, so anecdotally not scientifically, has been motivated by the pain of not changing being greater than the pain of changing (myself included).

I don't think you want your ex to wait until he's in his 60s. But that might be the case, if ever. Another thing you're not taking into account is earning secure changes everything, including shifting dynamics with other people. It really is a gigantic internal shift. You don't know exactly who you're going to be afterwards. Since I've earned secure and my FA ex didn't, we have drifted apart, especially during the pandemic. We tried to be friends, but his version of just truly platonic friendship is too inconsistent for my liking... other coping mechanisms of his seem immature to me and I don't feel very connected anymore or like we see eye to eye on important values. A few years ago, I was ready to marry him. Earning secure does not necessarily save a relationship, it may just shift it in other ways. That's just one example. Your relationship with your dad strengthened as a result of his change. I've had other relationships strengthen as well. And others fade out as my perspectives have changed. Everything is okay, I'm certainly happier now that I've changed and I've moved on from my ex into much healthier romantic relationships.

My point is, you have to keep yourself open to possibilities instead of clinging to this one specific hope because this hope is out of your control and can't be predicted. It's very personal to your ex's journey, and when he's gotten to wherever he wants to go, his ideal looking relationship or the kind of partner he can be still may not be satisfying or compatible for you. Clinging to this hope is actually a way for you to try to maintain control over yourself and environment in a situation that makes you feel powerless.

He may change, he may not. He may become subjectively better in your opinion, he may not. But after all these years of you trying the same tactics to get this relationship to meet your needs... you keep winding up in the same place.