Love and lust part 1

Can it be wrong to sleep with your partners best friend? Knowing that it is mostly lust that i feel, is it really being unfaithful.
Mostly, thats the word, if it was just lust then i would have no problem. But i have know this person for 10 years, we have grown close because of our positions, on a human level you could not hope to meet a better person.
We have not yet done it, but given the right circumstance i think it would happen, only our joint love for my partner stops us from engineering the correct situation.
It started with a kiss, just one. I saw that X was down and i said a kind word or two,a hug of thanks, turned into a passionate kiss, my first in years. My life turned. A meeting two days later to sort this out lead to another kiss, but longer. Now X fills my thoughts, my days and my nights.
My partner cannot see me as being attractive to someone else, and X as being very high sort after, these were nearly the same as mine, now i feel as if i have value and X looks even more appealing.
I'm not advertising this blog, if you find it then you were meant to. I would like comments, i cannot speak to anyone else, my partner is my best friend. I will add more when i can, this thing is my safty valve and as such will only be visited when needed.