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LW says this in the 1st paragraph: "I feel burdened, trapped by the overwhelming obligations of family and of keeping up appearances. The way I've tried to deal with these feelings is by seeing prostitutes."
LW, what you need is to decide whether or not you want to remain married and continue to keep up appearances vis a vis your faux family-man status. I think once you clue in your wife on your idea of "therapy", she'll help you on that one.
And yes, she needs to know. Maybe she won't even care! Maybe she'll divorce you so fast your head will spin! Maybe she'll ask to have a threesome! The sad thing is, you've been married to her for 20+ years and I'd bet you don't even have a clue how she'll react.