Can depression come and go?

I think i have always has this issue, feeling upset from time to time. When i am upset i focus on the things around me. I channel this negative energy away from me and into things i can associate with, blaming everything else but myself for being upset. I then find myself surrounded with excuses why i am not happy.

I am skeptical towards depression, although i know others have much more emotion to handle and they handle it in different ways, I don't believe that i am a victim to depression. I sometimes think that depression is somethign that you can cure on your own. Only lately i have been thinking that depression can be my excuse for the way i feel.

I am wondering if depression can come and go, or is it just my mental imbalance Something that i make up? The reason why i don't think i am a victim of depression is because I feel that i can 'cure' myself and that i have enough self esteem to know that i will be ok. But sometimes, it just doesn't work. I just get the feeling of being down, and i feel i can't do anything about it. I feel hopeless at the time that i can't 'snap outta it' like i can do so other times.

With time i get better, i seem to 'get over it' usually with the help of my boyfriend wanting to leave me. I make effort to change my mood when he says he can't put up with it anymore. We're happy together when i'm happy, so making the effort hasn't lead him to leave me yet. And i haven't left him yet, because i think that i can make the effort to control my mood. It has been 1.5 years now.

I also believe that i am selfish, that i have many great things that i don't take for granted. Because when i am feeling in a bad mood, i consider nobody else's feelings but my own.
I have tried to make an effort to control my mood. But i can't do it all the time. I am not into reading self help books, becuase i think all the help you need is from within. Its nice to have something to lean on or to have a reminder. I think that i am reluctant to think that i need a book to help me out, because peresonally i just think i haven't reached that level of depression.

I have been on birth control for maybe 7 months now. I use that as an excuse to my problems, but my boyfriend says its not an excuse.
I was on Diane (the pill) for 4 months and when i felt that wasn't working, i switched to implanon, the implant contraceptive. They both contain progesterone and i have been told that this hormone can cause depression. Could it be that? Even on sugar pills, i could be the same, i dont know.

I know this is a long post. THanks for your time to anybody who reads it. I would just like to know, is it just me? or have i got a problem that i actually have no control over?