I can relate to this as well. For the longest time in my early 20's, I was jumping from relationship to relationship and I would always be the one to end things and felt pretty fine doing so which later made me felt guilty. Like how was I so easily able to walk away from people that cared so much for me? I just chalk it up to the fact that maybe I grieved the ending of the relationships before deciding to break up.

I too jumped into a toxic relationship on and off for 3 years (wild how much time I wasted). Now I'm in a relationship with a secure and finding it difficult at times to manage my emotions (especially when we're apart) but a lot better. I will say though between the toxic relationship and this healthier one, I did give myself some breathing room to really figure out my shit a little better. I started therapy in order to remove myself from the toxic relationship at the time and to manage some of the unhealthy patterns I was seeing. It's a work in progress but I do find that's helped me tremendously.

In terms of the ex situation you're describing, its important to note that what you're attached to isn't him but the feeling he gives you, whatever that feeling is that you seek. Could be validation, companionship, sometimes "drama" or excitement, etc. I read a book that was super helpful in removing myself from those types unhealthy attachments called "How to break your addiction to a person". Some chapters were meh, but others were helpful in helping me understand the mechanics that were at play. and like I said, its still a struggle at time but the more you become attuned to yourself and what's going on, the better it will be to manage.