Welcome to Psych Central (PC). I am sorry you have suffered from sexual fantasies. Many people here seek professional help like the help of a therapist or psychiatrist. The problem with any fantasy is by focusing on it we give it a life that it does not have if left alone. Try counting when these arise and silently count the inhale as 1, the exhale as 2, the inhale as 3 and the exhale as 4...continuing up to 10 then begin at 1 again.
People at Psych Central find they can share these feelings and what they are going through with the confidence that people go through similar things and can empathize. So many forums are offered.
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You can also be an active member in other ways like supporting others in their questions, reading articles and posts
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