Starting exposure therapy and having problems

Today was the first day I finally properly did my therapy homework, which was to picture car accidents, one of my obsessive fears, in my head. I was able to do it for about thirty seconds (and started crying) before forcing the images out of my head and being unable to continue. Did anyone else have trouble even having the obsessive thoughts on command when starting exposure therapy?? I just couldn't deal with it. Any advice? It seems weird, but am I supposed to be forcing myself to obsess MORE?!