Am I cold hearted for moving on so quickly?

My boyfriend (now ex) and I broke up with each other on mutual agreement about a week ago. Our relationship lasted for around 5 months until I began to realize that he wasn't what I was looking for. So I suggested that we split up. At first, he was completely against the idea but I was able to convince him otherwise. Three days after our breakup, I posted on my snapchat story of me and this other dude hanging out and my ex saw it (yes, we still keep in contact). Then he texts me "Wow, you're already hanging out with other guys? Did our relationship really mean that little to you that it was that easy for you to move on? You're f*cking cold hearted." Was I in the wrong for moving on with my life in a short time span? Was I supposed to be depressed or something?

I'm 17 years old and my ex just turned 18 a month ago by the way.
Am I cold hearted for moving on so quickly?
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