After breaking up, I sent a message to my boyfriend saying that I missed you. I want to ask him if he misses me, too.

This is a good way to send. If he answers and thinks, you can send something happy. If you don't want to (don't trust him, because people will miss each other after breaking up), just answer with lovely words. Never talk about breaking up again, and then ask him out, just like when you were in love. You will forget to break up twice and then make up again. Send you a word, feelings are not easy to cherish, every time you say goodbye, it will hurt each other's hearts, and no matter how you adjust it, it will leave scars.