LDR ended 10 days ago, my ex is already trying to date

My LDR of just over a year ended around 10 days ago. I initially broke things off as the distance felt impossible.

We were very far apart (Texas / UK) and the only way of closing the gap was to get married and for my ex to take some big risks with his career moving. I was absolutely distraught and ended up reaching out to him to see if we could figure things out, I'm very much in love with him and thought perhaps we could still find a solution. He understandably doesn't want to and we've been out of contact since. He said he wanted to be alone.

However, I since had a feeling something was up, and found he'd posted not less than 3 dating ads on reddit 10 days ago. It stings to see it. Why do people do this? Also, any advice about the best ways to heal and move on - alone - would be welcome. We were engaged and had planned on getting married so it it's a lot

Thank you!