It clearly falls under 'new facts which were not included in the petition'
One can't simply travel beyond pleading
There is procedure for any such thing. else your plaint liable to be dismissed.
Ask for 'leave to amend pleading' and also mention that amendment is nothing but 'only addition' and 'no deletions at all' as such,
reasons:- 'only to augment and foster the earlier pleading as new facts are very material and relevant'.
Also narrate why you couldn't do the 'reasonable due diligence' and how the things were 'beyond your control' else your you would have never missed such a relevant fact right in the first place.
And make a very humbly worded plea.
Also ask for additional time to submit exam in chief owing to above as other party needs to give reply/objection to both of your 'leave to amend pleading' coupled with 'contents of amendments'
better make two applications.
there is no limitation and matter hasn't substantially progreesed, exam in chief also not concluded i.e. 'evidence stage' isn't even commenced.
Finally in your app, don't forget to state irreparable loss if you are deprived the opportunity to amend your pleadin to add new details.
Be ready to pay costs (costs aren't charged for reasons to be recorded and especially to plaintiffs)
Once that is through and your amended version is included in your plaint, then you can do as you stated.