Ask yourself what is the real reason you can't chuck the letters. From the sounds of things...I'd say there's a little unresolved feeling left from that relationship. Just my opinion. Obviously you know what they say...so really you don't physically need to keep them.
Analyzation over
...if you want them...keep them. Tuck them on a shelf in your closet. Simple as that.
Somewhere in this house are pictures of h's ex girlfriends and my ex boyfriends. We've been married 25 years.
I think my answer may be an complete non-answer....lol