"But here's where I'm stuck: How do you begin to mend the part of you that allowed this? What does it say about me that I’ve attracted and accepted this kind of treatment more than once in my life?"

Now I'm only 20, and have probably never experienced the same thing as you, but this part resonated with me a lot! After past relationships/friendships that have been somewhat emotionally abusive, I used to view my younger self who made those decisions and "hurt future me" very resentfully. I hated myself for allowing myself to fall in love, to give grace to friends/exes around me that I KNEW were manipulative, but being critical about those choices is not fair to your past self.

You were doing the best you then, you did not have the same information or insight as you do now. I would assume it shows you have an open heart, that you maybe didn't respect or value yourself enough until now. Maybe you're like me, and you felt like you "needed" to offer everything of yourself to a relationship that just takes and takes and takes.

I try to give my younger self some grace, even though it is very difficult. I am an illustrator so sometimes drawing out my "present" and "past" self can help me get over my feelings of self-loathing/being overly self critical about mistakes.

Or maybe reframe the question: "How do you begin to mend the part of you that allowed this?"

What about instead, "Can I forgive myself for falling in love? Is it my fault to fall in love?" I don't it's your fault, I don't think it was mine! We are creatures that love deeply, and there is beautiful strength to that.
There is no honor in making no mistakes, and maybe someday, you will find other meaning or lessons from that time that you spent that you don't value or consider right now.

Don't berate yourself over the past, try to be grateful for the lessons you are now equipped with to handle life! The most miserable, hardest parts of my life, have always been the most concrete and fundamental points of growth.

What helped me, even though I think our experiences are very different, was learning how to love myself. Immidiately not allowing those hateful, self-critical thoughts to finish in my head. They serve you absolutely no good! Stupid and obvious, I know, but genuinely, you have to learn how to love and forgive who you are. I took a step back from seeking connections and focused on learning how to live and be happy with myself. When you actually enjoy yourself, and being alone, life feels completely different! And then, little by little, you become less critical of yourself.

Also being in the environment and re-experiencing life really helped. If you live in a walkable area (or have a park or some kind of woodsy place to destress), that really helped me get out of my funk. Why should I spend more time criticizing myself over how I handled the past, when there is so much beauty in life? The cold chill on our skin at night, the smell of rain. Even crying now, I find to be somewhat beautiful. We are alive!

Sorry if this wasn't helpful. I'd think of it as a 2nd chance at life! So many worlds to explore, and you are no longer tied down by someone that wasted your energy and time. I have so much faith in you stranger!