Are you able to give other examples? Like, is it just expressing emotions? Or do you find yourself saying things like "I'm finishing work at 8pm" when you actually mean "I'm starting work at 8am"?
With statements like "I'm not scared", when we are scared, that can be common. And we can do it for all sorts of reasons. Like, this is me trying to convince myself I'm not scared, or I don't want them to know I'm scared, or I don't realise I'm scared, or, or, or...
It also happens when other emotions intervene and start telling us that "feeling scared" is bad or inappropriate. So, if you're feeling scared, your brain might be feeling angry/confused/guilty/ashamed etc about the fact that you're feeling scared. For example, "I feel scared on the bus, but my brain is telling me that it's stupid to feel scared on the bus, or it's embarrassing to admit that I'm scared on the bus", so we say what seems safer, or more appropriate, rather than express how we genuinely feel.
Super common example? "I'm fine." Almost NEVER means we're feeling fine. Fine is the kind of response that actually makes me raise an eyebrow an think, "Well that clearly isn't true!" Could be feeling really good, could be feeling really shitty, odds are that "fine" is flat out not true!
There's feeling an emotion, being aware of that emotion, and then expressing that emotion, and things can go in all different directions at any point in that process because of all the other thoughts in our head. Communicating our emotions accurately can actually be really hard. Emotions tend to come as part of a package, with a whole heap of thoughts, knowledge, beliefs, values, and other emotions, all attached to that original emotion. Complicated!
Which brings me back to the original question- is this an issue when you're communicating emotions? Or is it an issue with communication generally, even with emotionally-insignificant facts for example?