It sounds very hard. I don't feel open relationships are appropriate for couples unless there is a solid reason, frequently this reason is very old person and a younger person and an inability to function sexually. The reason I say this is because often things like this happen, where there seems to have developed a new relationship between your ex and this new person, especially because you say you are single, when you are not.
I feel your ex wanted to use you as a safety net whilst looking for another person. I suggest you look at why you would pursue someone so emotionally unavailable? Do you feel you deserve an equal loving partner? In my experience, people can push others away or enable their partners to push them away, when they fear they are not worthy of love or afraid of intimacy.
I advise you read the article I have reposted on intimacy and let me know if you have any ideas about how you feel about yourself.
All the best,