Cause of depression ?

Hey I'm Alex im 40 years old and i need your help to find out why i am deppressed. I have put togetehr some key events in my life which my be linked to the cause:

My life didn't kick off at the greatest of starts at the age of 4 my mother died leaving me in the care of my dad, sadly my dad couldn't cope and walked out leaving me in the care of my uncle.
Me my uncle and my uncle's friend lived in a house till i was 10 years old and my uncle passed away because of cancer i then lived with my uncles friend in america.
When I was 17 i got incredibly drunk with this girl and got her pregnant, and i became a dad. a year later when i was 18 we had another girl, again due to drink. since then i have given up drinking in large amounts.
My girlfriend left me when i was 20 leaving me with the girls, chloe, who is 3 was old aough to kind of get the idea of what was happening but cheryl who was 2 didn't have a clue.
at the age of 21 i mvoed back to england with the kids and lvied in a flat for 2 years before meeting my next partner.
All 4 of us (Me my Girlfriend Chloe and Cheryl) we were happy and the girls mum soon moved to england so she could visit her kids.
We got married and at the age of 25 we had twins, Holly being the oldest, Abbie being the youngest. i thought we would all live h ?appily but it was too good too be true. My wife began to abuse Abbie, to this day i do not know why she only absued Abbie, she lvoed Holly but it was as if Abbie had did something rong. The abuse was verbly and phsyicly it pains me to even think about it but after about 3 months of it, it got too much, and i took Abbie, Chloe and Cheryl and moved back into my old flat, I was devistated that i had to leave Holly but my wife wouldn't give her up. i divorced her and went to court to try and plead to have Holly lvie with me but i just didn't have the money to go back after loosing the case twice. My ex then put a restraining order on me to keep me away from Holly, and she left to America and told Holly i was dead. I still wonder how it hapend, i was foolish.
a few years later i moved to a bigger house and joined the army, i made it up through the ranks till i made it up to general and got to take lead of my own squad. When i was 35 i was called to american to take party in some training activity. it was only when i was tehre i got a phone call from cheryl (the smarter one ) tell me my restraining order had ended, so the first thing i did was went to see Holly. She was devestaded with me and her mum and couldn't understand what was happening. Anway she became clinicly Depressed and came to live with me.
The one thing all my kids have had trouble adjusting to is how much i love Abbie, i don't meen to but it apears to them i lvoe her more, i spend mroe time with her and more protectvie over here, i think it could be because what happend when she was a baby. Anyway I went with my squad to fight in Iraq, all my kids were very devisted by the sudden leave and were scared for me everyday. It came as a shock to me when the people i had trained with and grew up with for 4 years (my squad) were all killed in a roadside bombing, i was the only one who servived. those men were like brother to me. Now i lvie in a nice house, with a nice car, ncie jobtraining newbies how to hold a gun living with my fabulouse kids.
i have noticed and so have teh girls that i have become very low and deppressed, and i just i'm having a bad day, ive finaly come to terms with the fact i may have some sort of deppresion. Now im no doctor so can you please offer some advice and try to "decode" anything from my past
most of all thanks for reading this.
I AM HAPPY TO ANSWER ANY QUESTIONS YOU ASK!