How do I know I'm really here?

I have a problem discerning my existence in this world. I have many friends and a very active life, but when it all boils down to it, I can’t seem to keep a consistent grasp on my own being. am I really here? Are these people real? Is everything a figment of my imagination? Am I crazy and do I live a life within my own mind? I can only perceive things through my own being. When I’m alone, I feel like all that i’ve experienced is a dream and I have trouble retaining certain memories, like faces, sensations and feelings. I’m having a hard time with this. I need help?