Why am I rarely turned on?

My boyfriend initiates sex 99.99% of the time. I noticed that over time I become less and less interested in sex. Things that used to turn me on no longer do.

Honestly I might think about sex, but I don't feel anything down there when I do. I'm almost 22 and I want to enjoy sex more while I am young.

I have been getting increasingly sadder as time goes on (about life in general and how the world sucks - cynical, I know), so I am wondering if I am just depressed. I'm not as interested in other things I used to like as well.

Of course I have my good days where this isn't true, and many times after we get going at it I am more interested.

What are your thoughts and suggestions? I want to feel desire. Am I just asexual?