It’s harder to remember a lie. Because the only way to remember is to think back to when you told the lie, and remember exactly what you said. It’s easier to remember the truth, because you can just look around you, and check with reality.
If you lie to yourself, it’s harder to remember who you are and what you need to do. If you lie to yourself about what you want, for example, you will soon find that the reality of what you actually want is at odds with the reality of what you’ve actually gotten. Same for your abilities; you might find that you’ve bitten off more than you can chew and your real abilities are not matched with the reality of your challenges.
Better to tell yourself the truth, so that you are always working with reality.