How can I stop obsessing over a jealousy thought?

I saw a few years ago a comment from a girl on my husband's Facebook profile and it bothers me very much only now. She commented on a picture and he replied to her something that seemed a bit flirty. It was more before we even met , but I just want to be the one and only girl he was ever nice or flirty with even thought it sounds irrational or unrealistic. I keep ruminating on this for over 2 months now and I tried therapy, I tried keeping myself busy , doing fun activities etc. But nothing seem to help and I'm desperate and also a bit dissapointed in myself. I just don't understand why it bothers me only now , until now it didn't and I had period of times when I could manage jealousy really well. Is it possible that there's some root cause?

How can I stop obsessing over a jealousy thought?
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