I took NyQuil about 1.5 hours ago and I still feel terrible and cannot sleep. Can I take Benadryl (diphenhydramine)? If not, can I take an over the counter sleep aid?

Same thing. They all contain Benadryl (diphenhydramine) (or similar) and thus taking another brand is likely doubling up. It probably won't hurt but may or may work either. If this is a persistent problem, then you need to have better sleeping habits (avoid bright light in the evening, no caffeine after noon, no vigorous exercise near bedtime etc).