After a breakup, it can be tough to see your ex moving on. Whether you were together for a short time or many years, it's natural to feel jealous and even hurt when you start dating someone new. There are a few possible reasons why you might be feeling this way. Maybe you're not over them yet, and you're still hoping to get back together. Alternatively, you might be worried that they'll find someone better than you and that you'll end up alone.