Ethics aside, as a patient you are in a vulnerable position, so he should not take advantage of this, as you are there because you have psychological issues.
It is a known phenomenon for patients to fall for their doctors, it is because you are in need, he is delivering support and this can be mistranslated as caring for you in a personal way. They don't call it a doctor's "bedside manner" for nothing.
It doesn't mean because he is pleasant to you, and shows a face of concern (which is his job) that he is lusting for you or has fallen in love with you. As a man, of course it's possible he could be lusting after you (although one would hope he was professional enough to keep any such thoughts in check) but if so, he would only be wanting one thing out of any personal relationship. I don't think I need to spell out to you what that is. Whereas you, may have idealised him, thinking he is your knight in shining armour, will save you from a bad marriage where your husband is cold, unsympathetic and uncaring, and that it will all be "happily ever after". Life is rarely like that. You would get hurt, you would have imagined roses round the door where all he was imagining was getting you into bed. More married men would choose to have their cake and eat it and never leave their wife, than those who leave their marriage for the person they are having an affair with.
The other point is, that you say there is chemistry, but how do you know that this is not all in your mind? Perhaps you have latched onto him and he doesn't feel anything like you imagine. Or maybe he is aware that you fancy him and is flattered and his ego is being massaged so he entertains it but has no intention of allowing things to go further. If he is a handsome man, he could well be used to his female patients falling for him (although I would hope he has never abused his position because of it).
You asked how you should behave with him, well clearly as you are besotted with him you will never behave the same in your therapy as if you weren't, and this makes your doctor patient relationship likely to be less therapeutic than it is meant to be, perhaps far less. Your thoughts will always be on how you want him and your issues may not be addressed.
It just seems like a bad situation, I don't know what to suggest to you, I've told you what I think about the ethics side already, only you can make choices for your situation, I just hope you make the right ones.
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*Truth fears no trial*
DX AS & both daughters on the autistic spectrum