Ok, thanks for clarifying. It makes way more sense now!!

I think the issue happened because you were not "too honest", if I may use this oxymoron , and, therefore, the remedy you need is being "completely honest".

I am sure that if you tell the bf what you just told us - how the fact that your ex dumped you like a piece of trash for other girls made you feel used, and that, in turn, made you feel ashamed, and that - again, in turn! - made you feel uneasy about telling the current bf about the ex, he will, with time, see your POV. Other than being completely honest, there is no other remedy for this situation. Your thought process (felt like trash --> felt used --> felt ashamed --> felt like concealing the extent of the connection with the ex from the current bf) is not COMPLETELY obvious (but you understand it yourself - you wrote that you knew you should be thinking otherwise but still felt the way you did) - not COMPLETELY obvious - but it is logical/understandable enough to present to the bf. Just tell him the way it was. Also, there is this thing called verisimilitude, as I am sure you know very well, and oftentimes, it is exactly an unusual, illogical, non-obvious detail that renders verisimilitude to a story, precisely because it is hard to come up with something not-quite-logical when trying on purpose. In other words, the real truth does not always present itself as the most obvious, logical, straightforward, matter-of-fact story. So your explanation would - I hope!!! - appear credible precisely because it has the verisimilitude you need. Hopefully he will realize that!!