Trouble with diphenhydramine is it loses it's effectiveness, like most drugs do after time. You need to make sure you are utilizing proper sleep hygiene techniques. Get up at a certain time every morning, cut back on caffeine and eliminate alcohol after 6 pm. Napping during the day will mess up your sleep too. Don't spend time in your bed besides sex or sleeping.
You have to figure out what's keeping you stimulated, why you don't want to go to sleep. Set a time where you want to go to bed each night and don't watch tv or use the computer at least an hour before bedtime. Are there any other medications or supplements you're taking?