How come it is hard not to look for the ex's good qualities from a potential new partner?

It's not necessarily a bad thing to do this. Perhaps the things you are looking for are things that you know you like in a person, and just remind you of your previous partner. Despite this, it's important to also keep in mind what didn't work as well. You can think of your ex as a learning experience in what does and does not work for you in a relationship. I know for me personally when evaluating a potential partnership I will always make sure I look for what worked and what didn't. I hope this helps you find happiness with whoever you choose.