google 'narcissist' I bet your ex will fit the description, from what you say.once i discovered that my ex was a narc, I got a lot better at dealing with him, and was able to change my perspective from bitter rage to being able to laugh at his lies and bullshit.
Like you, for many years I did not seek maintenance, because i was scared of his reaction, and his treatment of me, but I am now in a position where his bullying and intimidation do not work. things are not perfect, and its easier now the kids are older and they can sort out contact (he has never agreed to a timetable and has them on a very ad hoc basis)
I only recently contacted the CSA again, (original claim dates back to 2000) so am yet to actually get any money, and to be honest, I doubt we will get much/any from him,given his previous deceptive techniques, (the twat moved abroad for a year telling the kids it was because I was taking all his money, all £15 a week!) but I will continue to pursue maintenance via the CSA until the youngest turns 18 as the kids deserve financial support from both parents, and having a 'private agreement' is pointless with this kind of bloke. any excuse and they will stop paying, as I have learned from experience!
get your CSA claim started quick as they are apparently going to start charging the parent with care to use the service.