To view your older messages (messenger chat history) please follow these 2 simple steps:
1. Go to Messages
2. Choose the thread of the friend you want to search. Scroll all the way till you find the message you want.
(Optional) If by scrolling you can not find the message, click on Options (the little cog) and then on Open in Messenger to see all ...
I have just one thing to say if that can help you out. Please visit Facebook and go to the activity log, if you remember the date or month filter it in the search and there you may be able to do some action. I don't know exactly but I think you may find a way there.
Let me know further.