Okay so it's kind of about what you don't have in your life and jealousy that your T has it? I think it's still worth thinking about why you're suddenly fantasiing about T in a sexual way as it says something about your work with her, even if it's simply that you feel safe enough to imagine what you want sexually with her. I think it's unlikely the obsession will vanish unless it's understood well and you're able to get those needs met in your life somehow. I hope things work out for you.