Iphone message hog delete

Ok to start off i have over 6 months of texts saved from my boyfriend (we text a lot) I want to delete the first 5 months of texts without deleting my recent texts ( few weeks worth) how can I do this without deleting conversation and or having to press each text to delete it? I have iOS 7 and it seems to be slowing my phone down. Is there a way to say delete all then I can tap to say don't delete certain texts?

Previous message until your finger cramps, then select every single message. There's no easy way, the guy who makes them probably never had this issue. I completely understand, I have a years worth of my girlfriends texts