Undoing Progress

My ex and I broke up over 6 months ago and I swore I was over him, but then I was at the store and an old mutual friend of ours that works there asked me if we were still dating and I told him no. But then I found myself thinking about my ex again and the old feelings came up. I don't exactly know why he left me because he broke up with my via text saying "I think we should see other people" and the typical "It's not you, it's me". How can I keep my progress of moving on from becoming undone?