I get asked this question a lot and over the years that I have coached people to attract back their ex, I've quite a lot of excellent advice on the topic. If your ex is open to spending time together (or going out on dates), keep it friendly but do not offer "Let's to be just friends" in the hope that your ex will think "Oh! You are so wonderful. I want you back!" Even with your other friends you are not romantically involved with, you didn't go up to them and say, "Let's be friends". The friendship kind of happened naturally over a period of time.
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