relationships after a break up

How do you know when you're ready to start another relationship after a break up?

I've been dating my bf for about 3 weeks & recently, my ex texted me for some closure. I was convinced that no feelings remained for him but I miss it. I dont miss him, just the feeling I had when I was with him which is a whole lot different than what i feel with my now bf. dont get me wrong, he's an amazing person but I often catch myself reminiscing