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).