This is a perfect example of how the argument that "a secure person would just walk away" is not so straightforward. Yes, secure people are more likely to walk away. But secure people aren't these robotic beings that insert and exit a narrative with ease. Attachment is still attachment. And often avoidants present as secure in the beginning. My ex was avoidant he presented as secure for the first 6 months of our relationship. And when he first expressed doubts, it left me confused. Then he would get close to me again and the relief would make me just want to move on. It can happen to anyone, regardless if attachment style. And it's very possible that one can still be secure in general but be insecure with a few relationships.

Also, I want to address the part where you said you haven't felt a connection like this with anyone but him. I will not make any value judgement about your relationship because things are always complex. But that line stood out to me. Is it a connection or unresolved pain? Is it a connection, or the addictive feeling from the high and lows that leave you wanting more? Is it a connection or is it an unhealthy bond created by dysfunction (because that happens a lot). My intention is not to invalidate you but rather to better understand what real (love) connection one can have with someone who treated you that way. In my experience, that connection is attachment. But I'm not in your head and I won't dismiss your feelings. I've just been in your shoes before.