you might want to talk to your local doctor as it sounds like it has been impacting a lot on yourself. depending on the type of depression there are different treatments etc. there are many different types of depression and one type is more chronic, i am not sure if this is you but its name is "dysthmia". there other types of depression called, "major depressive disorder", and then subgroups. maybe try Googling them to find out more information about them.
sometimes there is something that has happened in life that causes depression but sometimes it just happens for no reason other than brain chemicals.
If you want, i have found psychologist can help a lot when you get depressed and can help prevent relapse. just an idea.
best of luck
i agree with steffercakes