I have had cancer and an AN too! I am 27. I didn't have to have radiation or chemo though. Mine was in my eye and was surgically removed. So I am blind in one eye and deaf in one ear after my translab, among other issues.
I too have been diagnosed with OCD, although I feel like I have almost completely beaten it at this point. I used to drink a lot too. I started drinking to control my anxiety, because my OCD was completely out of control. Turns out some of the things I was feeling that were really aggravating my OCD were symptoms of a brain tumor. I saw a psychologist who helped me out a lot. I have had pretty severe depression for years, I have never been able to get help with that.
I'm not really sure what the link is between AN and depression. It seems very common. Are people with AN's more likely to be depressed because of the experiences they go through, or is the depression caused by damage to the brain during surgery? Both? The answer would be helpful for us all. I think you will never know why you are depressed and if it is related to the AN. The available treatments are the same no matter the cause though, so don't be afraid to get help if you think you need it.