I have been with my significant other for 12 yrs and he can't stop lying about everything... he says he doesn't mean to but unless I can prove to him that I know what he has said is a lie he won't admit to it and now he assumes that if I say something that I am lying[which I don't do]... I am at the point that if he says it is sunny out I literally have to look outside to see for myself... Is there an actual medical disorder to this? He has lied to people his entire life... I guess it would make him feel better to make up little things but it has now turned into big things... I do love him but I know it's not healthy for a relationship for me to always have to check what he is saying... How can I help him stop?

Antisocial personality disorder - Wikipedia, the free encyclopedia
Arm yourself with knowledge and see if he fits the pattern for antisocial personality disorder.

Wow that sounds like my ex which is my daughters father. We were together for 3 years and engaged for two, he lied to me from day one I just somehow was blind and fell for every one of them. There was a time when I was sitting in front of his house and I called him to see what he was doing and he said I am in the shower I was like um no your not because I'm sitting in front of your house and he's like oh sorry I'm on my way home from work and then 5 min later pulled up in his work van. He was just a hibitual liar and I could not get him to stop.

There are pathological liars who have a problem. I would think he would eventually grow out of it but since he is doing his whole life, I think he needs to see a therapist.