If you've got someone who is insecure, demanding, mistrustful, suspicious, jealous, and stalk-y (goes behind your back, pries, quizzes, cross-examines, snoops, hacks), there is nothing you can do. They'll 'find' 'evidence' and then start accusing you. If you respond to the accusations, you're now in the dock, and once you're in the dock, you'll be found guilty.
You're in the deal with them so you can take the blame for their insecurities.
They don't want the responsibility for their emotional lives, so there's no way you can love or argue them into taking responsibility.
They don't want to see things differently.
This is essentially the same as being in a relationship with an active alcoholic. Except you're their drink. You're what's giving them relief.
'I didn't cause it; I can't control it; I can't cure it.'
This can't be fixed. They can get well, but they first need to want to, and, whilst you're soothing their own guilt by being the scapegoat, you're blocking their rock-bottom.