My boss wants me to keep dating a prime client

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I am married and working as a marketing executive in charge of premium clients. The company has had a difficult period the last three years and now we only have a few client accounts left. Principally we are now dependent on this one client whose director I have been having an affair with. It started off casually with business meetings and now it is almost a relationship. I feel that I am being used here and it is not fair to my husband. My boss knows about this affair and he keeps asking me to do whatever it takes to keep that client. I want to end the affair but I think that will have consequences on me first as I might lose my job and for sure the company will close down thus putting six other people out of a job. Please advice on what I should do. I want to end the affair but I also want to keep this client. I just don't know how. Please advise... {Evelyn}