Why don't you show him your post, and any replies.
Regarding him 'forgetting' :
If he has had a recent conversation (in the last week) with his high school sweetheart about her about her struggles after breaking up, there is no way he would have forgotten - so he has lied to you. There are two reason he may lie to you : 1. He doesn't want an argument with you, because he knows you're insanely jealous; or 2. He's hiding something from you.
Regarding him not letting you see the messages :
He knows you have BPD, and if he knows anything about it, he should be showing you how trustworthy he is, not feeding you fears. Knowing it's an ex of yours - if it was all innocent, the first thing he should have done is openned the message boxes for you.
In regards to him deleting the messages :
That's fine (rather unlikely, but fine) - for if he does that with her messages, then he does that with everyones messages. Ask to see, not the contents of the messages, but his general inbox - which should have a list of messages from many of his friends. Within each friends inbox, the messages should have message dates. The number of messages, and message dates, will inform you if he deletes messages as standard practice, or not.
In General - if you are married to someone with BPD you have to make certain accomodations to make the marriage easier. If you have BPD and are married, you also have to make certain accomodations, which it seems like you are trying to do.
I'd suggest the two of you sit down with a doctor who specialises in your disorder (which may just be your family doctor) and sort out some ground rules for the two of you living with your condition...because you shouldn't be having this sort of fight over hidden messages to old flames.