in love with old friend

[youknow]

what to do, I have been friends with this guy on and of for 15 year's. we both have had partners and kids, recently his marriage broke down and he walked back into my life since that day I can't stop thinking of him. I get butterflies just thinking about him,he is everything I've ever wanted and more.......I have been with my partner for 10 years in that time we have only argued once, the sex has mediocre from day one ,I hoped it would improve with time but it has got to the stage that I have no intrest at all now .I crave for some passion, some excitment, I want a row just so we can make up!!! but i get nothing. I am 40 do I risk it all and tell this guy how I feel or do I coninue dreaming ?