Let me try to answer your question.
You are most certainly not "nuts" for having such feelings. You experience feelings of attraction, recognize them, and realize they are inappropriate, but have a hard time managing them. Males do this. You are doing a number of things correctly i.e., talking to your Pastor (though you may find that it adds
an additional, helpful barrier of inhibition between you and your stepdaughter if you actually told him the whole story); and you have talked to a counselor, who has provided at least one point of sound advice (NEVER tell your stepdaughter---to do so would likely make a real mess of your life and would unfairly burden her emotionally); and, you understand that acting on any of your feelings would be seriously inappropriate for you. So, you are doing some things that are helpful, but the struggle for you continues.
I'm not sure what your reference to anti-depressants in your post is about----you recognize that you may be significantly depressed (?) or someone has told you that you may be and suggested medication (?) What is the context for this reference, if you can say?
You may find it helpful to think long and hard about the following idea: You find that you are fundamentally unhappy in your marriage, or there is something missing that in all likelihood, you are trying to "fill" or compensate for. The focal point of your attempt to fill the gap in the intimacy domain of your life happened to be your stepdaughter, but you know this to be an inappropriate remedy for your needs.
You are wise enough to have already contemplated the idea of marital therapy to improve your marriage and repair the gap in the emotional intimacy arena of your life. Have you done this and in all honesty, do you want to try to improve this marriage, if it was possible? This is a question I'd like you to address?
When people feel they may be stuck or resigned to an unhappy situation they must learn to accept (a marriage they feel they cannot break up because their personal or religious values do not allow it), it IS important to think about making the acceptance more palatable through compensation. Literally, if I have to accept the fact that I'm going to be in this unhappy marriage, are there any possibilities for me to feel happier than I do----what compensation can I be allowed for having to accept this circumstance? So, some people who have given up and stay together in the face of what they feel are irreconcilable differences with transfer time and energy away from the relationship an home, having fully accepted that they do not want to divorce under any circumstance, and yet, they believe there are irreconcilable differences. They compensate by taking on new hobbies, are bit more selfish with their time so they can seek/derive more moments of happiness each week through some new activity (one guy bought a motorcycle and began planning cycle tours with other over-50 men in his area).
I very much believe every marriage can be improved, regardless of personal differences, if each spouse is willing to work on learning better communication, how to compromise better, how to be mindful of trying to be complimentary toward one another, and other actions. Changing actions and behavior is a very well documented mechanism for changing our feelings, incidentally.
Let me pause here and make sure I'm not overlooking an important aspect of your question, and give you a chance to answer my questions..............