ANSWER - Ok, you seem a bit confused. By Morgana, I'm presuming you either mean The Morrigan or you mean Morgan Le Fay. Morgan Le Fay is a sorceress from the Arthurian legend - more myth and legend than magical reality - so I'm going to assume you are referring to The Morrigan in your question. (If I'm wrong, please tell me more about Morgana).
Regular dreams on a single theme or relating to an individual, even a Goddess, do not necessarily imply that you are haunted. It is more likely that the dreams have a message for you that may have some bearing on your current life. If we think of The Morrigan and her connotations of war, death, fertility and conflict, can you perhaps think of any key ideas that are being brought into play here? Perhaps you are being offered guidance through your dreams.
I think it unlikely you need medicine to control your dreams but if you are having recurring images in your mind that you are finding difficult to control, you may benefit from talking them through with an experienced counsellor. Sometimes bringing dreams from the sub conscious to the conscious mind and looking at them objectively can reduce your fear of them. A therapist may be able to help you explore the metaphorical role The Morrigan is playing in your dreams and help you understand how to move on.
If you genuinely feel you are being harassed (and no, I doubt your sexual energy is playing a part in it....) then I would suggest you sit quietly, light a candle and tell her politely but firmly that you are not ready for her intervention at this stage in your life. It may well be something you will welcome one day but at the moment you would prefer to decline her attentions. Suggest she leave you alone for a year and then tries to make contact again after that time if she truly she feels she wants to work with you or has a message for you. I think it highly unlikely that any Goddess worth her salt will ignore a reasonable plea to be left alone.
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