hey..gu..
i used to have the samekinda feeling.. time to time..they appeared suddenly out of nowhere to me...and whenever i had this thing going on.. i thought life is useless..because one day we all have to die...and all that BS...its not a big deal..i guess it happens to a lot of people...its the spiritual side of you trying to come forward and take control of you...because u are subconcsiously tired of doing same thing over and over and over again...maybe you are sick and tired of working for  amimimum wage...or u are tired of studying hard..but u dont know what u want to be....or u are very stressed ..or you feel terrified working where u are workin now...if u work in a gasstation..these feelings are very common..try to live with them..with time they all fade away... let me know in what kind of situation they appear to you...my be ill be able to help you...