OP here, thanks to all the people that responded. The strange thing is that he seems to lie about most things, even really small, trivial things. It’s now at the point where I don’t believe a single thing he says. We have had issues before regarding infidelity on his side so I can somewhat understand why he’d lie if he had something big to cover up, but why does he lie over small things? I.e., he had a headache, I asked if he took panadol he said yes. He later admitted that he hadn’t. Why lie about that? Is it my fault somehow? What can I do? I just feel so lost.

Lying is pretty common human behavior, it normally stems back to the liar wanting to adjust outward reality, hide insecurities, avoid confrontation and a penalty, avoid hurting others, avoid hurting their ego or pride etc. I guess to get to the problem of lying you need to try and figure out why they are doing it, what emotion is behind it or is there a logical reason that they don't feel comfortable communicating to you. It's unlikely if you challenge or call out the behavior you will get a straight answer, because of the tendency for your husband to layer lies (cover lies with another lie) or resist communication as a defensive strategy. In terms of changing things, the approach can be simple or complex depending on why they lied. For example: Lying about a nappy change could be that he feels bad and guilty about making the mistake, or could go deeper. Psychology Today is a great website to read about methods to help a liar. Often most couples make the mistake of accusing, belittling or trying to catch their partner out, when that is not the route that changes liars only makes it worse. I hope this helps a little.

Lying is also associated with Aspergers - is he very sensitive ? Takes advice / suggestions as criticism? Flares up easily ? Does he have problems following new written and or verbal instructions ? Poor executive function/ planning? working Memory issues? / learning difficulties ? ( speaking from my own experience with spouse and son) it’s not intentional - but you have to be very tactful - they have to feel comfortable telling the truth - This is learned / unaddressed behaviour from childhood . Do a bit of research - also tactically show him research - lying damages your brain / amygdala
Love and kindness ( and a lot of research) will help him change ( if it is aspies) and improve your relationship

Yes my husband is a liar and he does it because he thinks everyone is stupid. I find it disappointing. I think he has got to the point where he believes it.

Not my husband but my ex did. All the time. That's why he's my ex