Hm. It's not just the fear, tbh.
It's that a controlling, coercive, abusive relationship is a process. It takes time, and by the time it's fully in swing, has you gaslit, confused, diminished, bamboozled, uncertain, destabilised, isolated, and subtly undermined in so very many ways, that you are often struggling to get through the day knowing what is true, what is going on, what is up and what is down.
There is guilt, fear, shame, confusion, depression, anxiety, brainfog and other emotions swirling about in there, often along with various bullshit stories about how he is a victim, needs help, can't live without you, and a lot of lovebombing in between. Plus a stream of tiny digs, suggestions that you are weak, defective, generally wrong.
There may be financial abuse, so that one is left with few resources.
Over time, friends and family have been turned away and pushed away, so there is little wider support.
If you've not experienced it it can be very hard to grasp how the dynamic works.
No woman should ever be made to feel worse for not leaving.
Only support. Only understanding. Only patient listening, and information when it's of use.
I do hope you get there, OP. Life is so much better on the other side. I can't begin to tell you.