Tinker, have you tried the pure parfum yet? Diorissimo is the most complex lily of the valley I ever smelled, and has in it other notes that do keep the scent longer. They are more apparent in the pure parfum - amongst them - boronia, jasmine, sandalwood, and civet, which holds everything together and make it last for a long time. I would suggest hunting down the pure parfum, and if you already have the EDT - layer it for an extra sillage.
I tried many LOTV scents, and Diorissimo is in my opinion THE BEST in any respects.