Past relationships continue to have hold over us even when we think we've adjusted to a new life or moved on. Because your X and you enjoyed some special times together, it makes perfect sense to feel sad and resentful when he meets someone else. Like Teresa @hopeful33250, I'm also happy you shared your emotions here – many experts say that the first step to feeling better is admitting or being aware of how you're feeling.
At times like this we often look at the past through rose coloured spectacles; you tend to forget all the reasons you and your X parted, right? Try and surround yourself with your loved ones and keep busy – keep sharing with the Connect community – and things will start looking better.