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 (http://en.wikipedia.org/wiki/Antisocial_personality_disorder)
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.