This is a very common manipulation tactic. People do it because they know putting themselves down is likely to make you want to reassure them about their good points (which, in turn, reminds you about their good points) The fact he is prone to blaming others is a red flag that it’s just one of his tools to manipulate you (people who have poor self-esteem are more likely to blame themselves for something that’s not their fault than to blame someone else for something that is their own doing)
A verbally abusive ex-boyfriend of mine used to pull the old “I’m worthless. I’m not good enough for you. You deserve so much better than me” card. He knew he’d be able to guilt me into falling into a protective, reassuring caretaker role if he did. Like you, for a long time I assumed he would have to think very little of himself to say things like that.
Well, come the messy end of our relationship, I’d had enough and the last time he ever tried the “I’m no good, etc. etc.” tactic- I agreed with him. I outright told him, “You’re right. I do deserve better than being insulted, belittled, and screamed at constantly. If you can’t treat me like a human being then, no, you aren’t good enough for me.” It was like I’d flipped a light switch and he immediately changed to ranting about how lucky I was to have someone like him and how, if I left, I’d never be able to find another boyfriend who would do the things for me that he did. Turns out his self-esteem was just fine and, like all manipulators, when one psychological ploy stopped working he just moved on to the next.
