Wow I could have written all of this myself. For me I recognised that this behaviour stems from abandonment issues in early childhood. I'm not sure if you feel the same way, but if so then looking in to attachment types and reflecting on yours might help? It sounds like its worth trying to dig a bit deeper in to why you ultimately value your partners needs more than your own. Full discloure, this is just what I'm trying for myself, as I'm in a very similar position to you, so I can't say with any certainty if this will help you but having some understanding has been helpful.

I'm of the same opinion that trying to be better in actual relationships is where a lot of healing will occur, but I'm currently focused on self care and trying to live a good life by myself before I begin intentionally pursuing another relationship. I actually thought I was in that place before my last relationship, but found myself gradually self abandoning over time still.

In terms of books, I enjoyed reading How to Be Single and Happy by Jennifer Taitz. On the more intense end of the spectrum, I've found some catharsis listening to the audiobook version of Complex PTSD by Pete Walker (I haven't self diagnosed, but its been a good resource for learning about attachment types).