Mental health and healing someone's state of mind are very broad terms. You may want to increase memory (in which case you have some very good suggestions in the above two posts). You may also want to
heal someone who's depressed, someone who's stressed, someone who suffers from hysteria or paranoia. The list is pretty much endless. Try and figure out what state of mind you want to heal. Narrow your search a bit. It will help you find a herb or an oil that's specific to your problem.