For me, lying came as easy as breathing. Due to FOO, I learned early on to fear people's reactions if you did not tell them what they wanted to hear. Life could be better, at least for a little while, for me. Speaking the truth meant instant punishment that, let's just say, was not something a child should have to endure. I carried that lesson into adulthood.
My BH did not know why I would lie about things like, forgetting to make a phone call or paying a bill and I never told him until after D-day when I was in IC why it was so hard for me to speak the truth.
I learned in IC a exercise that I put into to practice, starting with the A questions my BH had. When he would ask me a question, I would pause, remind myself that it was ok to tell the truth and that I could handle any reaction that speaking the truth brought. Also, I would tell myself that I was not responsible for other people's reactions to the truth, I was only responsible for how I responded to their reactions. (It was a long pause.) My BH knew why I took a long pause before every question, so he was not thinking that I was trying to come up with a lie and that helped me. What I was doing was retraining my brain to learn that it was ok to speak the truth, with the hope that eventually, it would just come naturally.
For me, it was easier to be truthful about all the A stuff than about small everyday bullshit. So I went back to taking a pause before answering a question.
About a year and a half ago, we had went to bed, I was reading and my BH had fallen asleep. About an hour later, I got up to take the dogs out and to smoke a cigarette before I went to sleep. (My smoking had always been a "Don't ask, Don't tell" type of thing our entire M.) So when I came back in, my BH woke up and asked me what I was doing. I told him that I had taken the dogs out and for a split second, I wanted to leave it at that. But for some reason, my mouth opened up and out popped "and I smoked a cigarette." He already knew that I had smoked when I was outside, he was just waiting to see if I would be honest about it, which thankfully I was.
But he asked me why was it so hard for me to tell him because if he didn't D me after D-day, he was not going to D for smoking a cigarette.
Big turning point because when he said those words, something in my brain just clicked and I have not lied about anything since, not even by omission. I think it was my brain rewiring itself and realizing that my BH loved and accepted me, faults and all. I had finally got it.
I will say that while it seems like a simple concept to just tell the truth for most people, from my experience, it was anything but. For me, it wasn't that I didn't want to tell the truth but that I couldn't. That probably doesn't make any sense at all. But basically, lying was a survival instinct that I had to learn starting at the age of 5 and I struggled with letting it go. My BH made it possible by reminding me that he was my safe person with any and all truth and our home was a safe place to be open and honest.
None of this may pertain to your WH, so I am not sure how much any of this will be of help to you. I just know that you have to want to figure out what makes you lie and be willing to actually work to make the changes needed. It is possible or instead of being R, I would be D.