How can I sometimes be so convinced this thing happened, and other times feel like it’s untrue. What is this? How do these memories occur?

Intrusive thoughts can latch on to anything. I have been dealing with false memory as well. It’s so heart breaking because you know this didn’t happen but your OCD is making you ruminate and overthink and review something to the point that you doubt even more. Some days I’m calm and even then I also start to doubt why I’m calm. It’s torture. But we will get through this. You are not alone in this. When you’re ruminating about a intrusive thought/false memory your giving it the spotlight and focus in your mind and so giving it more attention will then lead to strong emotions and that’s what makes it feel so real when in fact it’s just an intrusive thought. You have to try your best not to engage/ruminate with the thought ...easier said than done. I’m working on that in my ERP as well