I recently told my parents I'm nonbinary. They say that that's not real or valid and it makes me really sad. What should I do?

First of all, I want to say how big of a step that is, and I'm sorry they had a negative reaction. If you would like to try and convince them, you could explain why you identify as nonbinary, explaining the gender dysphoria aspect of it. As their child, you can ask for them to be more open-minded, or try stepping into your shoes. If they are still adamant in their ways, don't let that discourage you. You know yourself better than anyone else. Unsupportive parents are tough to deal with and work through, but don't let it discourage. There is a great community that will support you.