For me it was a combination of love-bombing and biological clock that got me in too deep too fast. Some vulnerability from not having quite recovered from my previous relationship too.
But also, as someone else said, they can just be very convincing for some considerable time. They are often capable of and willing to lie remorselessly, and to play the victim convincingly.
I only found out the extent of my ex’s deceit after 10 years and it was shocking to me; part of what stopped me seeing it earlier was my ingrained belief that if you tell someone you love them, you don’t fuck them over at the same time. It was just outside my scope of thinking that someone i knew, lived with and had children with was lying to my face regularly and faking being the victim whenever I questioned anything.
I had an idea of abusers as the kind of dominating, aggressive type, who would demean you by saying things like “You’re a fucking slag, you’d be nothing without me”. I didn’t realise they also came in a whiny version where they’d insist they didn’t recall saying the thing I was upset about, or complained that they “could never do anything right” or tried to isolate me by criticizing my friends for “being flakes and drunks” (neither true) or accused me of being controlling and paranoid while not being able to give any specific examples of this behaviour.
For my kids, I’m very focused on teaching them how to see abusive behaviours, encouraging them to trust themselves and their instincts, will never make them feel like a failure for ending a relationship, and will make sure the both have emergency funds that mean they can leave if they had to. Financial constraints were what kept me stuck for a long time, along with the shame of “failing”, fear of not being believed, reluctance to put my kids through a divorce and distrust that he would care for them properly when they were younger.