How do you know which of your behaviors are in your control and what isn't?

I was answering Dilbey’s question about whether you can control yourself and got to thinking about my confusion about whether some things I do are in my control or not, due to being addictive, or to having non-standard brain chemistry.

This question comes up for me in all kinds of situations. For example, getting depressed. Sometimes I feel like I could stop being depressed if I just decided to. But I don’t decide to. Is that my choice, or is that bipolar disorder? There are pleasurable things that create a dilemma for me, and raise the same question. Can I stop falling in love if I choose to, or is my need for love out of my control? Am I making a choice or am I being driven by things I can not control?

I wonder if any of you have faced this issue in your life. If so, what is the issue and what is the dilemma it poses for you? Have you gained control over something you thought you had no control over? How did you do it? How do you say no to the pleasures of giving in to that which drives you to do it?