Depression. The point of doing this?

How to know the difference between depression and sadness, I have symptoms like excess crying, bad food and sleep habits, sometimes I suddenly start panting without any reason I can feel my heartbeat, I am tired all the times even though I don't work, one day I feel like eating everything, and few times I starve for days, I can't sleep all night, I get angry on everything it feels like irritated. I tried to overcome all this thinking that I am lazy but when ever I try to do something I loose Intrest, I feel what's the point of doing this?for few days I feel natural, like I don't feel sad, and suddenly something hits me and I will start crying it goes for weeks, and then again it changes I don't feel sad for few days, it keeps on happening like Months infact year, lately when ever I cry it's too much to handle all I need at that point is to stop crying, it makes me feel so worthlessness at that moment I get angry, vulnerable. sometimes I feel stupid for my feelings, so I don't talk.