Op, I once had a partner who behaved the same way as you describe yours behaving.
I went from being a very sociable outgoing person to becoming a shadow of my formal self.
My relationships with my friends broke down and I lost a few friends due to being to afraid to meet up with them as I couldn't bear the interrogation I'd receive afterwards.
I couldn't even visit my sister as I'd be interrogated and verbally abused once I returned.
He didn't live with me (luckily) and I'd even be too afraid to go to bed early on a night as if he rung and I didn't answer I'd be accused of having another man round.
I had to ring him and tell him I'd be going to be because I was tired and even then he'd question me and not believe me.
He spat in my face once because a friend messaged me on Facebook and asked me to meet up with her to go for a drink!!
I was called a slag, a slut and a whore on a regular basis, when I'd done nothing wrong.
I got used to staring at the floor when I was out with him as if I even glanced at another person he'd accuse me of wanting to sleep with them.
The interrogations would go on for hours, with him asking me the same questions repeatedly as though he was trying to trip me up or something. However he couldn't trip me up because I'd never done anything. I was 100% faithful towards him and didn't have any interest in any other man.
When he'd finished questioning me he'd tell me I was lying and that I was a slag and a slut.
He'd watch through my window late on a night when I was on Mumsnet and tell me he'd seen me messaging/skyping other men.
It never ended and I know emotionally how hard it is to keep being interrogated and accused of being something you're not.
I ended the relationship and he was very abusive at first. He sent me a lot of texts calling me every name under the sun and saying that I must have met someone else. He simply couldn't comprehend that the reason I ended it was because of his extreme jealousy.
I am so happy I left. Since leaving him I've slowly rebuilt my social life and I can now invite people round for dinner or visit people without being afraid.
I can now go on nights out with my friends, I've even applied for and been accepted for a job I really wanted to do- something I couldn't have done when I was with him as he was too jealous.
He won't change op and like others say it doesn't matter one iota whether he's that way because he's mentally ill. What matters is your health and happiness.