It can be really intimidating to be all ready to win your ex back and suddenly see him in the arms of another woman. It can appear to you like he's completely left you behind, and like there's no chance to get him back anymore...but this often couldn't be further from the truth!
Rebound relationships have a bit of a bad reputation that's really not very well-deserved. The vast majority of the time these are harmless flings that only last a couple weeks (if that!) and serve a single important purpose: to make you feel better about life in general after such an emotionally draining and chaotic time as a rough breakup.
I think the movies have kind of made them out to be worse than they are, by always showing the person "right after the ex" to be the perfect soulmate, love-of-your-life person you were always searching for...but this is very rarely the case. Most of the time, people fresh out of breakups aren't looking for anything serious, and the benefit of the rebound is just to feel better about yourself and get over the worst of the breakup pain.
This doesn't mean that he's forgotten you...in fact, sometimes rebounds can only make him think of you even more, as being with someone who's either totally like you or not even remotely like you can bring up memories of the time you shared together. Whatever the case, these relationships are typically VERY short-lasting...95% of them never see a month's duration, and within them the bulk never see two weeks.