I have never been sexually attracted to him.
Then why did you marry him?
I travel overseas for work and last year began an affair with a colleague that lasted several months.
That's disgusting and you should be ashamed of yourself. But based on what you've described here, you're the type of person who doesn't consider how your actions affect others.
I can't really describe what it is that makes me attracted sexually to someone, it is not just looks, it is that connection and energy between two people.
Let me guess, your husband isn't an alpha male is he? You want to be dominated. Your husband doesn't fulfill this for you. It's not a mystery, there's no magic involved. You want a man who projects higher value than you. It's a very base, evolutionary drive.
It will be devastating for us both to separate
Incorrect. Your husband who put his faith in you will be devastated. He may even become so depressed that he becomes suicidal. But you will be fine. If you weren't fine with separating, you wouldn't have broken your husband's trust in the first place.
I wish there was a way to feel that spark, because then I would be content;
No you wouldn't. Because that 'spark' fades. It even fades in the most intense relationships with the strongest sexual chemistry. That is not something to build a future on. It's fleeting.
We have no children.
Why not have a child before you leave? Then you can trap your husband for child support and force him to pay for the new house you buy for yourself and Chad Thundercock.
(I) Love my husband
No you don't. By your own admission, your actions, including lying and deceiving someone who put their trust and 8 years of their lifetime into you, don't show any 'Love" at all. You are selfish. What you're looking for here, is for someone to tell you that what you've done and continue to do is justified. It's not.