You can't be faithful because this man makes you feel GREAT! It is all down to chemicals. Instead of concentrating on how great he makes you feel and how much you want him (we always want the thing that is hard to get) start thinking about how great your husband used to make you feel. Start thinking about where it's going to end up. Don't just think about how much you want this person, think about how things are going to go if you get this person. There will be three basic outcomes:
1) You will leave your husband (don't know if the OM has a partner or not) but he will leave his partner. You will get together. Maybe you will be ridiculously happy together. Or, in a few years you will have the same situation as you and your husband have now.Maybe you or the OM will look for a bit of 'excitement'.
2) His partner (if he has one) will find out. 9 times out of 10 when this happens the OM will drop the OW like a hot potato. You will get burned.
3) Your partner will find out. He will be hurt beyond belief and incensed. If he stays with you the relationship will never be the same again. He might also decide to dump you. You will get burned.
Where is this going to go?