I am beginning to think he has told me much bigger lies.
I'm sorry you are going through this Silverdash and can relate. During the breakup my ex's version of simple events that happened was so skewed from mine, it made me question my own reality. Then after the breakup, I found out about all the things she had been hiding from me relating to her mental health that I felt betrayed. Now I find myself wondering if there were other things she didn't tell me about. It's not a good road to go down but I can relate to having the same thoughts. Try to kind to yourself during this process.
It may not help much, but I found this article, which explains a bit about why this occurs:
Borderline Personality Disorder and Lying:
https://www.verywellmind.com/link-between-borderline-personality-and-lying-q-a-425190