Which supplement/herb/vitamin can help improve mental focus, memory, learning etc and is safe to take with ssris? and which should i stay away from?

Diet first: Consider the nature of your diet first. All the vitamins and essential minerals are found in "god made" food. All are essential. Supplements do not have the same effect if the diet is poor, particularly the sugar content.

Controversial: Supplements, herbs and vitamins effects on memory and learning dont need to pass the rigors of fda testing, so claims can be controversial. Some have stimulant properties and contain high doses of caffeine or other things that can increase blood pressure, etc. Some can be relatively benign. Its always a good idea to talk to your doctor about supplement use in terms of your own risks, etc.